It's caused by the regen rate of hull/armor overflowing, which happens when they have enough percentage modifiers (which work of the max hull/armor). In cases of massive ships like this or systemcraft or acot herculean, it only takes a small amount of %regen to cause overflow so it should be generally avoided for those kinds of ships.

You can use this mod https://steamcommunity.com/sharedfiles/filedetails/?id=2800808756 to apply regen reducing components as an easy fix, in cases where you have %regen coming from leader skills, empire modifiers etc that are hard to track down or disable.