Hi, @PYD 
You wrote:
I believe you're right! In an Ubuntu MATE 24.04 LTS ("Noble Numbat") VirtualBox VM, I activated the "On-Screen Keyboard" (available either in the "Ubuntu MATE Menu" -> "Universal Access" -> "Onboard" or by clicking om the accessibility icon in the task bar and by then sliding, to the right, the "slider" for the "On-Screen Keyboard" option). The on-screen keyboard then appeared on the screen but, when I mouse clicked on any key in that virtual keyboard, it disappeared. In my case, it did generate a ".crash" file named _usr_bin_onboard.1000.crash located in the /var/crash directory.
I've done several web searches and the most directly related bug that I've found is the following one ("Bug #2063041 “crashes immediately on pressing a key with python3.12 (Ubuntu Noble 24.04)” : Bugs : onboard package : Ubuntu" - https://bugs.launchpad.net/ubuntu/+source/onboard/+bug/2063041, that was submitted by the user "theofficialgman", on 21st April 2024):
https://bugs.launchpad.net/ubuntu/+source/onboard/+bug/2063041
You may want to click on the "This bug affects 2 people. Does this bug affect you?" link in that Launchpad bug web page (to add yourself to the list of affected users by that bug) and you may also want to add a comment in that page to report your experience about this bug.
On a related note: a few minutes ago, in that same bug web page in Launchpad, I've clicked on the link to add myself as one of the affected people and that had the result of changing the bug status: "Status changed to 'Confirmed' because the bug affects multiple users."
I hope this helps