their sniper garand count as heavy weapon.
If this was the case, then they would not be able to pick up more weapons unless of course they have 4 weapon slots, which seems unlikely, but if the stats could be procured for this then I'd believe the statement.
I think Pathfinders have a single sniper rifle as a slot item so they would drop BARs when reduced to less than three models. Unfortunately, you couldn't use the solution in the OP because they are only intended to have one sniper rifle.
It's not a bug in that the game is working as intended. For Infantry Sections, it's a serious balance issue given that they're supposed to use Brens to scale.
It happens for all units that have a non-droppable upgrade: G43 grens, PPSH cons, G43 Storms, Tommies, Pathfinders and so forth. I don't think it is working as intended. It resembles the old Guards who dropped weapons left and right, and that was fixed(to some extent). I doubt that all these units should have a higher drop rate than other units.