Curious, I went back to the save after landing on the beach, and had Shadowheart try to open the door. (It's DC 20) She managed to do it and then the comment was : "There, not so penetrable after all".
So, I guess they patched the door in some recent fix, as it doesn't seem to be tied to Astarion.
From my recollection, this has been the case forever.
The barrel nearby is 100% guaranteed to contain a Thieves Tool explicitly to give an attempt at picking the door.
I remember doing my first ever playthrough of the game and picking the door as a Tav and getting a comment about it.
