Russian infantry guns.

[DRG]

There's a pre set minimum range for those type of guns to fire indirect. You can see it if you try to fire indirect with the 76.2mm obr36 IG ( Russian unit 352 )you will find it will not allow any targeting less than 11 hexes from the gun

The other guns in question have "sabot" ranges to allow them to fire AP ammo and engage tanks but for that unit class there's going to be a simple line of code somewhere that is mixing up the sabot ranges with the minimum indirect ranges. That's why if you test this the obr 15 IG can target up to 3 hexes from the gun indirect but not from 4 - 10. It has AP "sabot" ammo with a range of 4 hexes and why the the 76.2mm obr27 IG ( unit 351 ) doesn't have any restictions at all as it has a "sabot" range of 60 hexes so it blows right through the 11 hex minimum for indirect

The bug doesn't affect normal howitzers which have a minimum around 20 hexes preset so it's just a matter of finding the bit that's throwing the IG's off

Don