I have noticed a couple instances where research options will be inaccessible because they are obscured by something else being on top of it. No other mods are loaded when looking at the tech trees.

The conflicting items are:


USSR: SU-14 obscuring BM-31-12 Andryusha

Britain: British Expeditionary Section obscuring Beaufighter 50kg bomb

Italy: Parachute Battalion obscuring SM 79 250 kg bomb