If you put 3 wither skeleton skulls next to each other, then place a t of soul sand under them AND 3 blocks to the right, the 4th head you place will trigger the wither spawning.
S=Soul sand
H=Head
1=1st
2=2nd
etc
H1 H1 H1 H3
S2 S2 S2 S1 S1 S1 =Wither Spawning
S2 S1 This graphic may view better on desktop
I have uploaded a YouTube video detailing what happened, it is possible to replicate it. Works for North and East, but not South nor West. I will upload both a screenshot of Before/After and the video aforementioned. It spawns the wither, when it should not. It seems to spawn in-between the two T's of Soul Sand, but the first patch is destroyed, so it is hard to tell.
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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.