(This has a bit of a story before the bug report, so that will be at the end)
So I, playing as America in an all-battlecruiser save, accepted a cruiser battle against Japan in this war we've been having. I get 2 of my battlecruisers; South Carolina, of the older Kentucky class, and Missouri, of the newer Massachusetts class. With them are 2 light cruisers, Olympia and Chicago, and 4 700-ton destroyers.
Immediately 2 enemy pre-dreadnoughts are spotted, and my battlecruisers engage, pouring rounds into them while taking light damage in return. Both enemy pre-dreadnoughts are badly damaged and my destroyers go after them for the torpedo kills when to the west (the Bs are to the east) the Japanese battlecruiser squadron is spotted.
This consists of 2 enemy battlecruisers each with a 6 x 13" -2 broadside. My ships are capable of taking this, especially Missouri, but they're running low on ammo, already partially damaged, and slowed down by the older South Carolina. The enemy also has various destroyers and a couple light cruisers.
My destroyers, although they have torpedoed and sunk the enemy battleship Mikasa, are currently separated from my formation
The position and speed of the enemy ships prevents my fleet from returning to base, and thus I set a course to the NEE. The enemy battlecruisers sandwich my formation, with one on either side.
My destroyer formation reorganizes and begins to try to rejoin my formation. I hope they'll torpedo one of the enemy BCs and get them to turn around. Instead 3 of them are quickly sunk while the 4th is crippled and has to return to base.
South Carolina runs out of ammo, followed 10-20 minutes later by Missouri. Olympia starts to take damage from the enemy battlecruiser and I'm worried she'll get torn to shreds, just like the destroyer formation, soon.
My battlecruisers are taking hit after hit, with the occasional flooding damage or speed loss. Night is approaching, but so is land which would trap my ships. The enemy battlecruisers and their destroyer escort are closing the trap. I have to turn around now and hope night hides my ships.
Missouri and South Carolina successfully turn around, but both enemy BCs follow them. Dusk sets in. We're almost out of this mess.
Missouri takes a hit knocking out electrical power.
Night sets with the enemy BCs close by.
Chicago fires a torpedo at Haruna and manages to get her to turn around. Missouri is still still. Kongo and her destroyer escort are right up next to Missouri, in a good position to torpedo her.
South Carolina turns around to allow me to keep Missouri, Chicago, and Olympia under my control.
Chicago and Olympia attempt to chase off the enemy ships nearby Missouri to prevent her from being torpedoed. They are nearly successful, but in the last encounter 1 torpedo gets through, causing massive flooding.
Unfortunately Missouri has to repair her electrical power first before pumping out the water. She begins getting dangerously close to sinking.
She restores electrical charge, begins to move at 10 knots, and shortly after starts limiting the flooding.
The battle ends.
In the results screen I am heartbroken to see that Missouri has sunk, despite the pretty good chance she had. Then I went to get a look at how close she had been to making it.
And this is where things get a bit buggy.
So Missouri here sank from progressive flooding despite having 0 flooding and not encountering anything after the torpedoing which would've caused additional flotation point attrition. Also of note, although it says she has 0 flotation points remaining (I checked this, not visible in the image), she was afloat for over 2 hours from her last "limits flooding" until her "sinking through progressive flooding." And because her current flooding is 0, then it makes sense that it should've been 0 for those 2 hours. No events like "torpedoed by a submarine," "struck a mine," or even ""rupturing a bulkhead" or "shipping water in high seas" seem to have occurred; they'd've shown up in the log.
Also of note is that since the battle likes to play the next 200 minutes out once it's over, 13:45, the time the ship sank, would be that last turn.
All I can think of causing this is that something with flotation damage is slightly wonky, which might allow a ship to remain afloat with 0 flotation points (or slightly less than that), but the game checks which ships are sunk at the end of the simulation by their flotation damage.
Unless someone can come up with a better reason as to why I lost my battlecruiser, I'm going to keep her after editing the save file so that she's still around, and the major penalties for losing such a ship in battle are gone.