Thanks to both of you for those suggestions - Sven, good advice, and it certainly can't hurt to make torpedo selection the last step. Threadfin, I was indeed in port when the calendar rolled over, so that must be the culprit. Your explanation of a default loadout makes sense - I suppose the calendar-dependent default is rigged that way to allow the introduction of more advanced types of torpedos into the game? Just a guess.
The funny thing about all this is, I sailed the patrol anyway, sore mouthing at first and convinced it was going to be a wasted effort. I ended up with one of the biggest hauls yet - 68,000 and some tons, including a passenger liner. Only 1 dud TII out of the lot.
Thanks again!