Not an issue... your boat's buoyancy changes depending on the depth you're currently in, AFAIK, not modelled in stock but modelled by the GWX team.
IRL it was necessary to keep a certain speed to mantain your boat at a specific depth, I don't know if the GWX went to far modelling that or if it is actually accurate. Problem is, that if modelling that feature, maybe you should be also able to set your dive planes to a specific degree, since it might be quite annoying to have to order "surface" just to order "current depth" a couple of seconds apart.
All and all, I think this is what's happening to you, of course a GWX team member will be able to confirm or deny this.
Hope it helps!
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