I can replicate this in 1.20.12. But it only happens when the wither is stuck in bedrock or when it is above a water area. It seems that the wither ignores any block below when performing the dash attack if it is in contact with either water or lava (flowing or source). I can't find any explanation to the bedrock one. So far it doesn't happen when fought on solid ground at least for me.
Edit: after trying for a while with endermen, I found an easy way to replicate the bug. And it renders the aforementioned theory obsolete.
Place a bunch of mobs with moderate health near the spot where you are going to create the wither.
Summon the wither.
Attack it until it reaches the 2nd phase (use strength 10 for example), and watch it use the dash attack just after you've done that.
You need to have mobs within 10 blocks of the wither to successfully replicate the bug, and it is advised to use mobs with moderate health so that they don't die before the wither gets to its 2nd phase.
It seems it has something to do with its first charged attack, I can't think of more possible explanations. A video was uploaded to show how it works.
Zifun
I can replicate this in 1.20.12. But it only happens when the wither is stuck in bedrock or when it is above a water area. It seems that the wither ignores any block below when performing the dash attack if it is in contact with either water or lava (flowing or source). I can't find any explanation to the bedrock one. So far it doesn't happen when fought on solid ground at least for me.
Edit: after trying for a while with endermen, I found an easy way to replicate the bug. And it renders the aforementioned theory obsolete.
Place a bunch of mobs with moderate health near the spot where you are going to create the wither.
Summon the wither.
Attack it until it reaches the 2nd phase (use strength 10 for example), and watch it use the dash attack just after you've done that.
You need to have mobs within 10 blocks of the wither to successfully replicate the bug, and it is advised to use mobs with moderate health so that they don't die before the wither gets to its 2nd phase.
It seems it has something to do with its first charged attack, I can't think of more possible explanations. A video was uploaded to show how it works.