Everything is impossible until the first time it happens.
https://van.physics.illinois.edu/qa/listing.php?id=2165
Neutral buoyancy and perfect balance is the perfect ideal for a diving officer, since it's NEARLY impossible to achieve the actual goal is "close enough for government work". Sometimes it can happen by accident, you get a good trim that's only slightly heavy, at a dead stop the sub sinks onto a layer with a slightly higher salt density - the sub is just a tiny bit heavier than the water it's in, but lighter than the water below it, so it just stops sinking and "floats" on the saltier layer.
Average US fleet boat displaced 1600 tons on the surface, 2400 tons submerged, so to get as close as possible to neutral buoyancy required 800 tons of ballast. Tweaking 800 tons with a gallon here and a gallon there would give you gray hair in a hurry, so "nearly impossible and not really worth the time and trouble" is probably the most accurate description of neutral buoyancy, so SOP was to get as close as possible with a reasonable amount of time and effort, then use the diving planes at minimum speed to maintain depth.
I don't have SH5, but the previous I through IV don't bother simulating that, you can stop dead in the water at any depth without any rise, sink, or change in pitch. I assume SHV treated the physics of flotation with the same casual attitude.