+1, I've encountered this too.

I had a problem with the beast collar disappearing before, although it was different.  It was also in pre-LH days, but it might still be relevant.

I had two beast collars on my sovereign.  I used one of them and successfully tamed a beast.  While I only used one, both of them disappeared from inventory.  I didn't notice until I went to fight another beast (fortunately I noticed it before going into combat).  

It was too hard to try to reproduce, and it seemed like something that would be very rare (having two beast collars), so I never bothered to report it.  I only mention it now as it might be related.

I've seen this too.  If you select an item in an inventory, and then try to 'cancel' use of that item (by clicking on another item or in some blank part of the map) the item disappears from your inventory.