SUMMARY

On removing a single external tool definition, all of them are removed on next start.

STEPS TO REPRODUCE
1. Open preferences > external tools (haven't used those much - so they should be pretty vanilla with defaults)
2. select entry "git blame" in the tree
3. click on "remove/delete" (translated back to English)
4. the node "git blame" (and only this node) is removed from the tree
5. check the menu extras > external programs looks fine (only one entry is missing as intended)
6. shut Kate and reopen

OBSERVED RESULT

The sub-menu extras > external only offers a single entry for configuration. And in the configuration, the tree is gone - completely blank except for one node "uncategorized".

EXPECTED RESULT

Only the selected entry shall be removed, not all of them!

SOFTWARE/OS VERSIONS

Operating System: Kubuntu 22.04
KDE Plasma Version: 5.24.4
KDE Frameworks Version: 5.92.0
Qt Version: 5.15.3
Kernel Version: 5.15.0-40-generic (64-bit)
Graphics Platform: X11
Processors: 8 × Intel® Core™ i7-4700MQ CPU @ 2.40GHz
Memory: 15.5 GiB of RAM
Graphics Processor: Mesa Intel® HD Graphics 4600

ADDITIONAL INFORMATION

Symptom seems very similar to bug 453272. Only I didn't type or edit any of those external configurations. Also I don't have a folder ~/.config/kate or ~/.config/kate/externaltools

Comparing with my backup, I found a new file was created as ~/.config/kate-externaltoolspluginrc
It has only three lines:
---
[Global]
firststart=false

---

I renamed that file to kate-externaltoolspluginrc.bak and relaunched Kate - that brings back all the external-tools definitions, including the "git blame", that I tried to delete.