I for one cannot continue to play a game where exploiting a map construction prevents the land-lease from reaching the USSR.
Actually it seems to have become a common tactic. Axis subs and navy backed by a maritime bomber in Norway. But there are lots of counters you can try (practice in hotseat):
- turn off the convoy yourself for a few turns, keep the money and lower the NM impact on Russia from raiding
- invade Norway
- smash through with RN (upgrade AA and ASW and send a carrier in fighter mode)
- smash through with RN and US navy
- diplo Finland or Sweden
I don't think it is a map error or an exploit. There are lots of other bottlenecks. Yes, maritime bombers are powerful, but they don't like bad weather or strong AA.