If you are going to throw "JPA compliance" around, of course you are going to be expected to list the section in the JPA spec that says that. I'll save you the time... there is none, JPA is silent on this. And previously Hibernate did not document either behavior specifically. Meaning it was completely undefined.

Appending is clearly the safest option. Given appending, over-writing can still be achieved by truncating the file before you start. Hence appending should be the default.