Completed event of Entry is raised on Tab key when Text is not empty.
Entry on ContentPageCompleted eventTab keyCompleted event only raised when Enter key pressed.
Completed event raised when lost focus i think.
This is probably because the NSTextField is set up to send its action on "end editing"; what you are asking is for it to be sent on "enter only". Unfortunately, while this can be easily accomplished using the Xcode nib editor, I cannot immediately see a way to do it from C#.
I am going to look at fixing the Completed event invocation to be the desired Enter/Return key only on macOS.