I don't know where this was documented and if it is documented at all.

But I'm almost sure that before 5.0.3 the only way to write JPQL query for enum with AttributeConverter was:
SELECT e FROM Entity e WHERE e.type = 1
It not worked if I wrote it like this:
SELECT e FROM Entity e WHERE e.type = com.sample.EntityType.TYPE1.

After 5.0.3 the only way is this:
SELECT e FROM Entity e WHERE e.type = com.sample.EntityType.TYPE1

So the problem is not that 5.0.3 breaks some accidentally supported way to write JPQL queries. The problem is that it breaks the only way to write JPQL query with enums with AttributeConverter. Maybe break word isn't right here. Formally it fixes it. But it's still breaking change.

I already gain an understanding of this change but I think other people who will upgrade from 4.x to 5.x will come across this trouble if it will not be part of documented breaking changes.