Yes, it's a bug with the autoclutch. When you lock a wheel while downgearing, it seems that the engine cuts out, but this has no effect on the car's behaviour.
You can erase the bug easily by having a quick small push on the throttle each time a gear goes down.
Actually, the mod is not designed to be played with automatic gear changes, and the best piece of advice I can give you is to shift by yourself and even to switch off the automatic clutch (using a button as clutch is possible). You will have a more realistic driving experience, and you will avoid this particular bug!