I found this when trying to log in to my user account through Connect's remote input. My password includes an uppercase letter but when using the show button on the password field, had it had the one letter as a lowercase instead of the uppercase. I tried it with compose send and with the regular keyboard input -- it had the same result. This makes me unable to log in to my account with KDE Connect.
Thank you for the bug report! The issue involving the compose send functionality in my testing seems to be restricted to Wayland, and works fine in X11.It is being tracked in bug 483730. If you find that it also does not work for you on X11, please add a comment to that bug report. I have also tested the case of letters sent with direct send instead of compose; I can confirm the issue with Samsung Keyboard on Android, but not with Gboard where upper case letters are properly sent on both X11 and Wayland. Can you post some information on the specific virtual keyboards you experienced this with?
I am unable to test X11 functionality. I can confirm that using the Gboard does allow for capital letters with direct keyboard input, but still none with the compose functionality. Thank you for the help and workaround for this issue, as my use case is now satisfied.