Not a bug but rather different layers of interference in your case it sounds like the culprit is the change in depth causing water to flow past the array faster. try waiting untill your depth is stabilized and reduce speed. Also if you are below the thermal layer sound propogation may either increase or decrease dependent on the layer type.(you may hear better or worse). im not sure how realisticly the layers are modeled i know it was this way in DW.
Hope this helps
