This actually doesn’t solve my problem, but found another issue with kirby builder and the latest kirby 3.4.3.
It is currently not possible to save a kirby builder block with required fields (without when condion) with kirby 3.4.3.
It seems the behavior only appears for public and unlisted pages. Switching to draft solves the problem. Switching to kirby 3.4.2. solves the problem as well.
Note: Required fields (with when condition) work as expected.
Probably it has to do with the new 3.4.3. kirby feature:
Required fields with when conditions now can be left empty, if the when condition does not apply…