Found this interesting - I had a similar problem with CardDAV contacts (the thread on that).
As far as I could tell, my contacts did get “broken” over time (bad Nexcloud update by me while self hosting or something else, not 100% sure).
Radicale (haven’t tried Baikal) is more liberal in terms of what it accepts, so it would take it all as it is.
Betterbird email client is similar - it would work with it.
Em Client was the first (and only one) to complain.
Had to fix my contacts (some manually, some by importing them to an up-to-date managed Nextcloud instance), and then it all worked.
To avoid the problem I’m using managed Nextcloud for the time being.
Contact updates from Em Client and from Iphone seem to not be causing any problems for now.
Not sure if this helps, but it was a bit scary and quite frustrating until I figured it out.
Relja