Windows build number: 10.0.18362.0
Windows Terminal version (if applicable): 0.7.3291.0
Entering fullscreen only works on two of my three monitors.
On the third one terminal just maximizes and shows nothing but black/gray and a bright white titlebar.

My machine has two identical monitors, but _fullscreen_ only works on the primary monitor. Fullscreen on monitor 2 fails per the screenshot above.
I have three identical monitors.
Two of them connected with DisplayPort and one DVI.
The secondary DP craps out when entering fullscreen.
Reminds me of an issue over at PowerToys:
https://github.com/microsoft/PowerToys/issues/611
Are those displays connected to different display adapters, or to the same one? What's your graphics hardware situation?
Videoprocessor : GeForce GTX 1050 Ti
Videomodedescription : 1920 x 1200 x 4294967296 colors
DriverDate : 05.09.2019 02:00:00
DriverVersion : 26.21.14.3630
All connected to same GPU
GetSystemMetrics = 3
GetMonitorInfo OK
EnumDisplayDevices OK:
DeviceID = \?\DISPLAY#GSM59B4#5&e062d6f&0&UID4353#{e6f07b5f-ee97-4a90-b076-33f57bf4eaa7}
DeviceKey = \Registry\Machine\System\CurrentControlSet\Control\Class{4d36e96e-e325-11ce-bfc1-08002be10318}\0010
DeviceName = \.\DISPLAY1\Monitor0
DeviceString = Generic PnP Monitor
GetMonitorInfo OK
EnumDisplayDevices OK:
DeviceID = \?\DISPLAY#GSM59B4#5&e062d6f&0&UID4356#{e6f07b5f-ee97-4a90-b076-33f57bf4eaa7}
DeviceKey = \Registry\Machine\System\CurrentControlSet\Control\Class{4d36e96e-e325-11ce-bfc1-08002be10318}\0012
DeviceName = \.\DISPLAY2\Monitor0
DeviceString = Generic PnP Monitor
GetMonitorInfo OK
EnumDisplayDevices OK:
DeviceID = \?\DISPLAY#GSM59B4#5&e062d6f&0&UID4355#{e6f07b5f-ee97-4a90-b076-33f57bf4eaa7}
DeviceKey = \Registry\Machine\System\CurrentControlSet\Control\Class{4d36e96e-e325-11ce-bfc1-08002be10318}\0011
DeviceName = \.\DISPLAY3\Monitor0
DeviceString = Generic PnP Monitor
EnumDisplayMonitors OK
WMI report:
Active: 1
WeekOfManufacture: 9
InstanceName: DISPLAY\GSM59B4\5&e062d6f&0&UID4353_0
YearOfManufacture: 2016
ManufacturerName: 71 83 77 0 0 0 0 0 0 0 0 0 0 0 0 0
ProductCodeID: 53 57 66 52 0 0 0 0 0 0 0 0 0 0 0 0
SerialNumberID: 54 48 57 78 84 83 85 52 88 56 54 50 0 0 0 0
UserFriendlyName: 50 52 69 66 50 51 0 0 0 0 0 0 0
Active: 1
WeekOfManufacture: 9
InstanceName: DISPLAY\GSM59B4\5&e062d6f&0&UID4356_0
YearOfManufacture: 2016
ManufacturerName: 71 83 77 0 0 0 0 0 0 0 0 0 0 0 0 0
ProductCodeID: 53 57 66 52 0 0 0 0 0 0 0 0 0 0 0 0
SerialNumberID: 54 48 57 78 84 90 78 55 51 54 48 53 0 0 0 0
UserFriendlyName: 50 52 69 66 50 51 0 0 0 0 0 0 0
Active: 1
WeekOfManufacture: 9
InstanceName: DISPLAY\GSM59B4\5&e062d6f&0&UID4355_0
YearOfManufacture: 2016
ManufacturerName: 71 83 77 0 0 0 0 0 0 0 0 0 0 0 0 0
ProductCodeID: 53 57 66 52 0 0 0 0 0 0 0 0 0 0 0 0
SerialNumberID: 54 48 57 78 84 74 74 52 88 56 50 56 0 0 0 0
UserFriendlyName: 50 52 69 66 50 51 0 0 0 0 0 0 0
Both of my monitors are connected to separate DPs. Both monitors using NVIDIA GeForce GTX 1060 6GB GPU. Both monitors using native resolution (1920 x 1080).
I'm seeing this on one of my monitors, plus different behaviour on a second.
The two 2560x1440 panels (one of which is primary) fullscreen fine.
The 2560x1600 panel to the right of primary displays like the screenshot above.
The 1920x1080 panel fullscreens perfectly to the top half of the display, with the bottom half matching the screenshot.
All panels are DP off the same AMD FirePro W5100.
Same problem for me. I have an nVidia NV300 (one connector for two DVI monitor). The display shows a strip on the left but the rest is entirely grey (dark theme).

similar to https://github.com/microsoft/terminal/issues/3710#issuecomment-559074329
but with more cols.
hardware: laptop
second screen conected via hdmi to intel graphic adapter.
primary screen also register as conected to intel graphic adapter.
main screen resolution(left):1366X768
secondary screen resolution(right):1920X1080

I have three identical monitors, and full screen behaves differently on each. On my center (primary) monitor it works fine. On my right monitor, the terminals are only sized about 20px wide (see Aarklendoia's screenshot above), but the app does take up the whole monitor. On my left monitor, the app takes up the whole monitor, but now the terminals are sized larger than the app. I'm using panes, and what happens is my right-most pane is pushed offscreen, while my center and left panes are visible and look somewhat normal.
I have a laptop with one external monitor connected through DVI. When my laptop screen is set as the main display in display settings, full screen works on both screens. However, when I set the external monitor as the main display, fullscreen only works on the external monitor and on my laptop it shows similar results as image from OP. None of the displays are scaled (100%). I hope this helps.
My setup:
Windows build number: 10.0.18363
Windows Terminal version (if applicable): 0.7.3291.0
Laptop with 4k display with 150% scaling on an Nvidia GeForce GTX 950M GPU;
Two external monitors: 1080p display with 100% scaling connected through an HP 3005pr USB 3.0 dock.
On the main (integrated) monitor, fullscreen works perfectly; on the two external monitors, fullscreen looks like this:

Environment:
Windows Insider Preview 19025
Windows Terminal 0.7.3291.0
I have two same screens, and Alt+Enter works well in each screen if I don't move terminal from
the first screen to the second. Once I move it, I will get the this bug!
Assigning myself as followup to test whether the in-PR fix helps this.
I'm having this happen on my Laptop, 2 displays connected via Docking Station over USB-C work fine. The 3rd via HDMI from Laptop results in the full screen bug reported here.
I'm seeing the same issue. I have two identify monitors but the second one (the one where the failure occurs) is in rotated by 90 degrees.
FWIW, this started working for me after upgrade to WT 0.8 on both machines where I previously experienced the issue.
works for me now.
Thanks!
Most helpful comment
My machine has two identical monitors, but _fullscreen_ only works on the primary monitor. Fullscreen on monitor 2 fails per the screenshot above.