There is a much greater probability of gyro errors and circle runners in RFB than in stock.
Earlier in the war, deep runners are pretty much a given.
Also, if you you have anything other than 0 rudder, it will throw off your shots as the change in heading could not be compensated for in real time on WWII boats.
In your post it seems you are talking about a limited number of shots. If so, there's no way to tell if you are experiencing a bug, or if you had bad luck with the gyro error probability.
If you are talking about a large number of shots doing this with none of them going straight, then we need some more info:
1. Model of torpedo
2. Type of boat
3. Manual or Auto targeting
4. Date in the game
5. Turning or on a steady course
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"There are only two types of ships- submarines...... and targets" Unknown
"you wouldn't catch me on a ship that deliberately sinks itself"- comment to me from a surface sailor. 
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