The range at which hopper can pick up items from is inconsistent, because as far as I have tested they are able to pick up items which sit on top of a block which has a hit box as high as 0.875 blocks (soul sand) which is placed on top of a hopper.
However hoppers are not able to pick up items which sit on top of the hit-box of the stand of a brewing stand which is placed on top of the hopper (which is only 0.125 blocks high).
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Still a concern in 1.7.4/5 and 14w08a

Is this still a concern in the current Minecraft version 1.8.1 Prerelease 3 / Launcher version 1.5.3 or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.

Reopened, thanks
The following video gives idea of what causes this bug: https://www.youtube.com/watch?v=qEi-tKrxDNk
It basically is due to the fact that minecraft does not check the area above hoppers for entities when a block is placed on top which has an inventory. So since a brewing stand has an inventory, this means that hoppers do not try to detect whether there are entities above it and only tries to pull items out of the brewing stand instead.
The hopper in this case is looking at the stand and not the item drops.
In otherwords...
This is NOT a bug. Works as intended!
When a container is put on top of the hopper, it is assumed the hopper is dedicated to this container.
Confirmed on 1.5.1.