I confirm this problem!
I always have to be very careful to stay above a certain depth if I want to keep my speed below 100 rpm. Otherwise you cannot convert your descent and you will be crushed, unless you increase speed, alarming the escorts. There is a point of no return somewhere at very low speed. This point for the type VIIb/c is closer (150-170?) to the surface than with the IX (170-190?). I am surprised that there are other kaleuns who seem to have no problems with this phenomenon.
I am using GWX2 and no mods that changes anything other that sound and graphic files.