Well it is a bug. Slowly sinking to the Earth's center is not normal real-life behavior. Sure, at periscope and you hit all stop and you sink a little. Either you'd drop down to a neutral buoyancy depth some small amount or you'd drop far enough that the engineer in charge or trim would do something about it.
Better behavior is settling a random (5-20m) depth below commanded when not under speed and some fluctuation over time as the engineer tries to fine tune the trim being very worried about breaking the surface.
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