I double click on an entry, select the "Custom fields" tab and have 2 lines:
X-CUSTOM-KPILOT-MODIFIED and KPILOT-RECORDID
On the right of the value each line has a red minus icon, which I assume is to remove the line. However clicking on it does not do anything and I see no other way to get rid of a line.
It does not matter where the contact is stored. Even when I try to remove a line while creating(!) a new contact, before having clicked on the OK button (the contact is not created yet), it does not work.

The version is in fact 5.17.2 on

Operating System: openSUSE Tumbleweed 20210618
KDE Plasma Version: 5.22.0
KDE Frameworks Version: 5.83.0
Qt Version: 5.15.2
Kernel Version: 5.12.10-1-default (64-bit)
Graphics Platform: X11
Processors: 4 × Intel® Core™ i5-6300U CPU @ 2.40GHz
Memory: 15,5 GiB of RAM
Graphics Processor: Mesa DRI Intel® HD Graphics 520