It's about conditional chain command blocks.
How to reproduce:
1. Place a repeating command block, and a conditional chain command block (set to always active) connecting to it.
2. Insert into the repeating command block:
, and set it to always active
3. Insert into the chain command block:
4. Place a block on top of the repeating command block, it gets destroyed instantly but the chain command block does not run. (Expected to see [@] a in the chatting bar)
Why I think it is a bug:
1. If the "destroy" keyword is replaced by the "replace" keyword, it works fine
2. If the /fill command is replaced by /setblock command and still use the "destroy" keyword, it works fine as well