Below in the picture you'll find first how the relation is done. Second how it is used in subpath and finally which error this produces.
This structure used to work with v5.2 from where we migrated to a this new version. The relation is created here with llblgen, not in the sql server.
The prefetch path element at index 1 in the passed in prefetch path for root entity type 227 is meant for root entity type 288 which isn't a subtype of 227. This means that you've added a prefetch path node to a Path of an unrelated entity, for example adding OrderDetailsEntity.PrefetchPathProduct to a prefetch path for CustomerEntity.