This one is very weird and hard to explain, so I might attach a video if no mods/helpers are able to reproduce it. These are the steps I took to reproduce the issue
1) Make a wither formation but don't place any skulls.
2) Place a dispenser like this:
fill it with wither skulls and activate it once
3) Place a dispenser three blocks to the right and one block below
4) Make a wither formation below the new dispenser, fill it with skulls and activate it once
5) Now remove the wither formation and press the lever again
Dispenser put the wither skull in the air, which isn't normally possible at all:
6) Now remove one block from the wither formation:
Dispenser on the right can no longer put a wither skeleton skull in the air
I couldn't get it to work if second dispenser is placed elsewhere, but there probably many more possible locations where it happens. Someone needs to check the source code for why it happens.