Anvil items vanish on merge when anvils fall at least 2.5 blocks onto non full cube block

The bug

Anvil items vanish when merging after an anvil
When multiple anvil blocks start falling and fall at least about 2.5 blocks onto a non full cube block like a button or a slab, each anvil will correctly produce an item but the item merging is not correctly done, resulting in one anvil item in total no matter how many anvils actually dropped.

How to reproduce

1. Build a setup as shown in Setup.png
2. Remove the redstone block
   → It looks like 2 anvil item dropped, but merged to a single anvil item