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:
/fill ~ ~1 ~ ~ ~2 ~ minecraft:air 0 destroy
, and set it to always active
3. Insert into the chain command block:
/say a
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
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The same thing happens with /fill and replace:
/fill 38 218 12 38 218 12 trapped_chest 3 replace
That actually DOES replace the chest, emptying the contents (desired behavior here), but the output says "0 blocks filled", causing chained conditional command blocks to do nothing.
Very annoying and counter-intuitive to have to resort to using tags instead.