Note i have tried this on 20w45a snapshot and 1.16.4 release. I Have also tried this on a server and offline single player. 

So i been trying to convert wither heads from a wither into an explosive i can use for a wood farm, and while trying to figure things out i discovered that the wither heads will always destroy the wood in a certain position and NEVER destroy them in another. for this i made sure that the block in question was taking the head directly and it wasn't a surrounding block.  

I only change how close the block was. I assumed it was something to do with tick rate and how it's determining collision so i made sure to try offline singleplayer to make sure this wasn't a server issue. but the same thing occurred.