exactly what the title says, probably a minor detail valve devs forgot to add when they made minis repairable
master branch.start_tf2.bat (or start_server.bat) with no custom launch options.map command ingame).

This is intentional
// do not show low health for mini-sentry
case BUILDING_HUD_ALERT_LOW_HEALTH:
case BUILDING_HUD_ALERT_VERY_LOW_HEALTH:
bShowAlertTray = pObj->IsMiniBuilding() == false;
m_pWrenchIcon->SetVisible( bAlertTrayFullyDeployed && bShowAlertTray );
break;
Found in game\client\tf\tf_hud_building_status.cpp
@RealHoriuchi that was because when the gunslinger was first added it couldn't repair mini sentries,it could only refill their ammo. this is why if you empty a mini's ammo you get the notification.
edit: basically valve forgot to remove that piece of code after gun mettle
edit 2: grammar
Although minor, definitely wouldn't hurt to alter this.
I would argue that this is still a feature, not a bug mainly due to the disposable nature of mini-sentries, particularly that they are meant to be spammed (ie. destroyed and rebuilt). Repairing minisentries is not that common and having the indicators in those kinds of situations would get spammy/annoying very quickly.
You could just make it a setting to toggle the old behavior but that seems like more effort than it's worth.
I would argue that this is still a feature, not a bug mainly due to the disposable nature of mini-sentries, particularly that they are meant to be spammed (ie. destroyed and rebuilt). Repairing minisentries is not that common and having the indicators in those kinds of situations would get spammy/annoying very quickly.
You could just make it a setting to toggle the old behavior but that seems like more effort than it's worth.
The indicator is there, just bugged (at least for me)