I expect the vent is pretty small so most of the air displaced from the tank will come back up the filler. Where is that in relation to the cabin entry?
Sounds like a poor connection between the filler cap on the deck, and the tank. Have you had a chance to check the connecting pipework ? Are there any traces of diesel in the bilge after a refill ?
Is it a single tank, or does it lead to a smaller "day" tank with a separate breather ?