A possible second reason just came to my mind. I bought BS1 in the US PSN Store, BS2 in the European (German) store. Perhaps there is a difference between those versions (at least relevant for the save game import).
Like I said, the bs1 save game is stored on my account (same account I want to play bs2 with). With PS4 it's possible to buy a game with a different account than playing it.
To
summarize:
BS1: bought with account 1 (US store), played and saved with account 2
BS2: bought with account 2 (EU store), going to play with account 2
All on the same PS4.