Some update from my side, since I don't know whether this was previously already observed or not.
Further testing revealed that my units get resupplied as long as their parent formation has supply elements - them being actual part of the HQ unit and not a seperate unit under either HQ or the supplied unit. What's interesting is, that in the latter cases supply units still get used up but don't actually resupply anyone. And I am using supply vehicles, not infantry.
So in other words; if my brigade HQ has supply trucks, then units under it will get their supplies but if the brigade HQ has a subordinate supply company, they won't resupply anyone but still get used up.
So the solution for now seems to be simply to design "one-time use" HQs with massive amount of supply elements...