As the Russians I compelled a Finnish surrender, but for some reason the Axis are still getting supply in this port that was hiding the Tirpitz.
Is this a bug, or to be expected?
"War is never a technical problem only, and if in pursuing technical solutions you neglect the psychological and the political, then the best technical solutions will be worthless." - Hermann Balck
Yes, as they still hold the port they will receive supply from it. Place a land unit there to attack the Tirpitz and it will either soon be badly damaged, sunk, or forced to flee.