Otherwise put, the issue here is that the datetime type used by hibernate for mysql / mariadb for the "timestamp" hibernate mapping is not migrated automatically to datetime(6) upon upgrade of XWiki.

It's not very clear what XWiki versions/setups did have a datetime type without fractions(so that they can be impacted by the lack of migration) but we know a couple of things:

• it seems mysql / mariadb specific, other db engines have fractions in the types mapped to hibernate's "timestamp" type by default (e.g. postgres)
• I reproduced it for servers with MySQL 5.7 and 5.x connector, including XWiki 11.10
• I did not reproduce it for servers with Mysql 8+ and 8.x connector, starting from 12.10

However, I didn't test on an instance of XWiki 12.10 with mysql 5.x because I didn't have one. I don't know if it could happen as well or not.