Couldn't find this reported, not a huge bug but this is what happens:
When you have 2 quick bars, and you have set a filter on position 1 on bar 1, example, an inserter. The slot is empty.
On pos 1 on bar 2, you have another item, a repair pack. Bar 2 is active and you take repair pack into your hands, you switch to bar 1 and then you press Q to return repair pack to your quick bar, it will be on the inserter-filtered slot (pos 1 bar 1).
If you want I could make a video of it, but I think it's clear enough
