SUMMARY
With a regex with 4822 bytes length, the search was performed multiple times OK on 20.04.3. Reopening or rerunning the search made the whole desktop environment either unresponsive or made it crash to return to the graphical login.
After disabling the compositor activating the search function does not make KDE crash, but the search window is as wide as the search string is (= VERY wide).

STEPS TO REPRODUCE
1. Entered a regex search term with 4822 bytes length into the search window.
2. Closed the application.
3. Reopened the application.
4. Opened the search window (Ctrl-F or via graphical menu).

OBSERVED RESULT
The search windows is as wide as the search term is.

EXPECTED RESULT
The search window is as wide as the screen and either the search term field has a vertical scrollbar or the search string is wrapped field with more height.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Debian Bullseye (Testing)
(available in About System)
KDE Plasma Version: 5.17.5
KDE Frameworks Version: 5.70
Qt Version: 5.14.2

ADDITIONAL REQUEST
Could not find where the search term is stored on disk and could not find that in the documentation.