Sinking other subs?
 

So I found an island above the UK (forget its name) Docked there were 8 unsuspecting U-class submarines. So naturally I decided to sink them. I fired a torpedo into one and it was destroyed, but the sub started to go crazy. It was doing flips underwater and eventually landed on shore where it continued to bounce. The same thing happens with every one of those subs. Additionally I don't get the tonnage for the ship being sunk as small as they may be.

Is this just my game glitching or is this a common problem?

that happened in sh4 too when you would sink the mini subs they would fly around like crazy bounced all over.

most likely just the same bug from sh4 was ported to sh5

maybe increasing its mass 10 or 20 fold might make it less bouncy

It is pretty funny to watch though :D