This issue tracker is for improving the CPython distribution.  It is not a tracker for other software, or for general help with running Python.  For the latter, try
https://mail.python.org/mailman/listinfo/python-list

Thonny is 3rd party software with its own tracker, although I suspect that the problem is with your upgrade Chromebook/Linux Bullseye.  While running Python on Linux on PCs is supported, I do not believe Chromebooks are included; there is certainly no Chromebook buildbot for testing CPython listed on https://buildbot.python.org/all/#/builders.  If there is a Chromebook Python list, you might ask if tkinter works with Bullseye for other users.

IDLE *is* part of CPython.  What specific release of Python are you running.  This is the first thing IDLE prints when it starts.

If you are running a somewhat recent release of 3.9 or 3.10, then continue.  To see whether there might be an IDLE-specific or ever tkinter-specific problem, run the CPython test suite from a command line with "python3 -m test -j0 -ugui" and report results.  (Or 'python ...' is 'python' works to run python 3.) If it passes except for IDLE tests, start IDLE from a command line with "python3 -m idlelib".  Get IDLE to quit/crash.  Then report any error info printed in the terminal.