I see that this issue was posted 2 years ago (MC-5056). However, I was moving around items in my inventory when the GUI position changed and the item under my cursor was no longer the item I was placing individual items into. The buff left my character because I had moved outside the influence of my Beacon and the beacon buffs timed out.

If this is not a bug, this issue should at least be a Feature Change Request.

I can't believe that this is acceptable to other player, especially if they are in the heat of a PVP battle or about to be attacked by a mob and need something from their inventory only to have that item under the cursor change position.