At the moment, the datastore emulator is not able to act as the "real" Firestore in Datastore mode.
To be specific, transactions do only allow ancestor queries. The emulator itself is capable of behaving like Firestore in Datastore mode, using the argument --use-firestore-in-datastore-mode - but one cannot set this in Spring.

Datastore Docs: https://cloud.google.com/datastore/docs/firestore-or-datastore#in_datastore_mode