I think there is a bug in SH5 that means AI surface vessles can remain locked on you until you save and reload the game (even with IRAI). Of course a bug like this is a game breaker (because you can't know it's happened or not unless you spend hours trying to evade, only to realise all your efforts were in vain), and that's why I don't play SH5 anymore. As it's the only half-decent WWII subsim out there, even after 4 years, it's a bit of a shame
By-the-way, how this shows up is, when you use T.C. and sail off in any direction, you'll see an enemy vessell tracking you from several kilometers out, and when you stop, they circle in one position, forvever! Sadly, I don't think this bug was ever fixed by TDW or anybody else.