See title, vid of it in action:

Nothing in logs whenever this happens. It seems like this happens more/only when the player breaks the shaft holding the belt?

Can confirm this is happening, and I have some more details from my own testing: the items will be lost if you break the "origin" shaft of the belt, the one that you first right-clicked with the belt item. Breaking the second shaft (the "endpoint") or any other belt section regardless of shaftiness, is safe and drops the items as expected.