This is not a really bug, rather it's an inconsistency with snares.
Axis Faust = minimal AoE + 'damage all in hold' false = no damage to units inside halftracks.
OKW AT riflenade = low AoE + 'damage all in hold' false = minimal damage to units inside halftracks.
USF AT riflenade = high AoE + 'damage all in hold' false = minor damage to units inside halftracks.
AT nades = medium AoE + 'damage all in hold' true = high damage to units inside halftracks.
Now where the snare hits is important and is propably why you thought Pio's don't receive damage inside the 250 while they actually do, it makes positioning of the 250 important:
AT nade hits the front of the 250 = no damage to models inside the vehicle.
AT nade hits either side = high damage to all four models.
AT nade hits the rear = high damage to the two models in the back of the 250.
To the patch team: because AT nades are the real outlier here, I'd propose setting 'damage all in hold' to false as the most easy solution to this problem.
I imagine this been like there is no kind of guideline so we end up with so many inconsistencies across the board.
Probably no one up to this date bothered to check it. Should PM modteam if it's possible to fix now or later as it SHOULD only be a true to false change.
PD: never undersestimate how unrelated things can bug out.