Hi,
I've set a cart validation group to check the customer email maximum characters (as an example since seems to have the same behavior with all fields).
100 characters => passed validation;
110 characters => did not passed validation (my template is not outputting the error messages);
300 characters => checkout retrieved an exception;
This seems to happen when the field value length exceeds the DB field maximum characters.
I've tested this on 9.3.13 and 9.4.7.
https://www.screencast.com/t/67Y1yzZ06kA
BR, Mario