alright, i have a lot more specifics now.
1) The reduced range is the range of the brit's mortar emplacement (non supercharged).
2) Only Piat sappers cause this. Regular and advanced repair sappers have regular range.
3) with piat sappers manning the gun, if one is killed (2 on the gun) the range is reverted to the normal range.
4) if a squad of 2 piat sappers mans the gun, it has normal range.
I hope this helps.