Certain Items Unreachable

I have multiple items in a wooden box, stacked six boxes high, and locked down. I can manipulate (grab, drop, move) small items in/out of this box but larger items, once placed into the box, cannot be manipulated whatsoever. When attempted, a gump appears; "That is out of sight."

IMAGE:
Items in red circle can be dropped into the box, but cannot be manipulated after.
Item in green circle can be manipulated completely. (tested with various small items)


Is this a bug? If not, please explain.

also happens with harps :/

I’m assuming there is a weight threshold at work here but haven’t tested any further.