Well that's probably because of your mapping and hibernate not being smart enough about the expression.
If you map the ids as normal fields in PotDates and use these directly, Hibernate should be able to avoid the join.

Excuse me for the off-topic but is there any chance Hibernate can be taught to understand such expressions in future versions?

The problem is that the query is generated dynamically and joins are based on metamodel. So if we have two or more entities with "ids as normal fields" and these ids are part of composite, we will not be able to match which id field in one entity should relate to which id field in another entity.