Hmmm. Playing at 1:1 ratio and realistic, I have pretty much the exact OPPOSITE experience as you
The choppahs seem to be pretty danged good at getting into my general position and putting some death fish in my general vicinity (and requiring me to do lots of crazy maneuvering to evade death).
Even when I manage to finally get back to being undetected, they will continue to lurk my area for a long time.
I had one encounter off Norway coast where I foolishly ventured in a bit too close to shore so I was pinned with only about 400 ft of depth to work with.
I had managed to sink about 4 escorts and a couple merchants and I got greedy and came in to the shallows looking for one of the escorts which was "hiding" (sitting way over there motionless). Since he was a long way off (detected him with radar if I recall) I decided to use harpoons, and for some reason that summoned a helicopter to my location very quickly.
Net followed quite a desperate series of attempts to evade his torps and escape. I hope it was more than one aircraft the game was sending because it is hard to believe a single chopper could carry that many . . . In the course of all this, I discovered some ways to "cheat" (sort of . . .) even with all the settings on "realistic" (e.g., no 3d without full Identification of targets).
For some reason, that "No 3D" doesn't apply to wrecks, it doesn't apply to the owner ship, nor does it apply to the sea floor nor aircraft. So, in spite of aircraft not appearing on the tactical map, I was able to figure how where he actually was by using wrecks/sea floor markings to figure out where I was relative to him and thence get away from his search pattern.
Eventually he started moving off to the west, southwest and I presume he was returning to his secret undersea lair in the middle of the North Sea?
I had seen some past complaints by users that the aircraft exhibit unrealistic protracted loiter times, and in particular that choppers whose mother ships have been sunk should pretty much bug out immediately. I cannot comment on the latter, as I don't know if that chopper(s) would have been one which necessarily came from one of the escorts I sank or not. But it does seem like they have impressive loiter time.
The situation in the campaign at this point was Norway still in allied control, so I'm not sure what an ASW helicopter whose mother ship was sunk in that locale would do. If another viable mother ship was handy then that would make sense, but short of that, I suppose trying to get back to friendly territory via the shortest route (overland) might be the only option? Else just kamikaze it into an allied location or radio in to the allies and request to be taken prisoner and that you are going to scuttle your aircraft near the coast??