Block icon disappearing when using a lead on a fencepost

I used a lead on a horse, then scrolled through my inventory to a slot with another block in it.

I then proceeded to right-click a fencepost.

This block quickly appears, then immediately disappears and the lead pops up on the fencepost.

The block has disappeared from my inventory, but is still there.
For instance, if I were to right-click when on the empty slot, the block gets placed, etc.