I have SH4 1.4 and have never seen the MBT gauge shown in the OP's pics, so I assume those are from 1.5 or the Uboat expansion. The other gauges aren't random, they accurately show depth heading and speed, but the physics model for buoyancy and balance is so oversimplified it shows either a complete lack of knowledge for how it actually works or complete apathy. So if they added a ballast tank flood gauge I would expect it to reflect that same ignorance or apathy. When underwater all the main (external) ballast tanks are usually completely full, but if the safety, bow buoyancy, and negative tanks are empty the sub will have positive buoyancy and rise to the surface.
Many "experts" will tell you that you can never have any air in the external ballast tanks when submerged because the external pressure will collapse the tanks. Completely forgetting that for US subs the flooding ports at the bottom are open to sea and air is compressible, so automatic equalization of internal and external pressure. "Put a bubble in number 7" to compensate for compartment flooding aft was commonly done, but in this game you either have full or empty ballast tanks, no in betweens.
"must take on even more weight to descend further - which contracts my present understanding of buoyancy"
I think you mean "contradicts" rather than "contracts", but trust me, your understanding is better than the designer of that gauge.
