This has been my experience as well -- both ways in fact. I used to fly in and out of Slovenia a lot and until my flight from GRZ to FRA was scanned I couldn't get my UA BP for FRA - EWR. Once it scanned it would magically appeared in the app... First time it happened I was nervous but it always worked out.

This happens quite a bit with different airlines -- when I would connect in LHR from UA to GF I was NEVER able to get a BP in advance -- I would always have to go to the transit desk post security and then they would print.... Frustrating but common these days....