Doing a eos-factory-reset on a upgraded installation restores the desired behavior.
The issue only occurs on systems which have been updated from 3.8.7 to 3.9.0.
Checked on Ubuntu 20.10 which also uses GNOME 3.38 ➞ it works there as expected.
Checked if there’s something in the GNOME Keyboard component in Endless Source Tree different from the official GNOME Repository ➞ they are on par ( ).
Checked if it has something to do with Endless GNOME Extensions, so i disabled them all to get a Plain GNOME 3.38 Desktop ➞ same Result, does not work as expected.
For other Applications which expect input, it does not pop FYI.
However, it does not input the touched Keys into the appropriate location.
When the OSK is enabled, it pops up when GNOME Shell expects input (like Alt-F2 to run a command or the Google Input Box on the Desktop).
Back on the original Topic regarding the On-Screen Keyboard.