This is likely related to the overall pathfinding behavior the game uses for most mobs. Often times, if an entity (a villager, for example) has a destination in mind and is moving towards that destination, often times that entity will not account for a sudden change in location, such as teleportation, and keep moving towards their destination, not stopping for a quite while until they arrive, or realize they can't reach it. Something similar might be happening with the Wither in this instance.
Can confirm that this is affecting version 1.20.81
Here's a video i made further testing the bug.
According to my testing, this is how the bug works:
Starting from the skull in the northwest direction, the game checks the area to the southwest of the skull (a 4x4x5 area) for another newly placed skull. Regardless of what block it's placed on and the orientation, placing a new wither skull within this area will trigger the summoning.
No longer affects version 1.20.81
Tested this using a potion of decay while wearing armor. This report should be marked as resolved.
This bug still exists, but due to its obscurity, i doubt it will ever be fixed.
Still affects version 1.19.71
This bug still occurs in 22w46a
This bug still affects version 1.19.30
Still affecting version 1.19.30
Still affecting 1.19.30
Still affecting version 1.19.30
This is probably a side-affect of MCPE-131325, where it only appears to be raining. I tested this and it works just fine in the rain.
Still an issue in version 1.19.30
This is still an issue in 1.19.30
Still affecting 1.19.30
Still affecting 1.19.30
Still affecting version 1.19.30
Please reopen either this issue, or my issue. The bug is not a problem with the playsound command, but rather with the music itself, and has not been fixed.
Still affects 1.18.12. Probably the latest 1.19 releases too
Can confirm, as it still Affects 1.18.12 - 1.19.0
Can confirm that this is affecting 1.20.81