@MH17
In Tactical\Overhead.cpp:

void RemoveStaticEnemiesFromSectorInfo( INT16 sMapX, INT16 sMapY, INT8 bMapZ )
{
	if ( !bMapZ ) // Battle ended Above-ground
	{
		SECTORINFO *pSectorInfo = &(SectorInfo[SECTOR( sMapX, sMapY )]);

		pSectorInfo->ubNumAdmins = pSectorInfo->ubNumTroops = pSectorInfo->ubNumElites = 0;
		pSectorInfo->ubAdminsInBattle = pSectorInfo->ubTroopsInBattle = pSectorInfo->ubElitesInBattle = 0;
	}
	else
	{
		UNDERGROUND_SECTORINFO *pSectorInfo;

		pSectorInfo = FindUnderGroundSector( sMapX, sMapY, bMapZ );
		pSectorInfo->ubNumAdmins = pSectorInfo->ubNumTroops = pSectorInfo->ubNumElites = 0;
		pSectorInfo->ubAdminsInBattle = pSectorInfo->ubTroopsInBattle = pSectorInfo->ubElitesInBattle = 0;
	}
}

which is called at the end of the battle in CheckForEndOfBattle()

// Kill all enemies. Sometime even after killing all the enemies, there appeares "in battle" enemies in sector info
RemoveStaticEnemiesFromSectorInfo( gWorldSectorX, gWorldSectorY, gbWorldSectorZ );

Maybe it would be enough to add

pSectorInfo->ubNumCreatures = 0
pSectorInfo->ubCreaturesInBattle = 0

to RemoveStaticEnemiesFromSectorInfo() to fix the bug?

This still leaves question why this information is not updated when creature is killed.

pSector->ubNumCreatures++;
pSector->ubCreaturesInBattle++;