In a recent game, noted a minor hex ownership issue where regions captured by a former belligerant remain in their possession even after they quit the war and they have been occupied by another country. See attached screenshot.
What happened was Bulgaria had surrendered and is now neutral but those regions of Serbia that they had captured remained Bulgarian even after being occupied by the advancing Greek army. There are a number of regions that this applies to, all outside the initial Bulgarian border including Aleksinac, Pirot, Leskovic and Vranja. Those Serbian regions under Greek control, Carev Kuk, Ishtib, Doiron and Dobro Polje were taken from the Austrian and Bulgarian forces while the latter was still at war and so it was working as designed as long as Bulgaria was in the fight.
This is not a really big deal and may be edited out of the save game file (imposing Serb or Greek ownership) if desired but note that the Greek HQ is still in supply so I am not entirely certain what is happening here. If the regions were truly neutral one would think that it should be out of supply or even isolated but this is not the case.
Apologies for the relative text wall for a minor and easily modded matter but if regional ownership remains after a national surrender, it might be worth looking at a fix. If enquiring minds want to know, this game ended with an Entente minor victory at the end of the turn.
If this is working as designed, no problems as long as friendly supply may be traced through these regions.