No seed in particular does this, and as long as its above easy it works. Your gamemode also doesn't affect it. I've had it happen in all gamemodes except adventure (because I haven't tested it in adventure)

Only crashes the host of the world, not anyone else in it.

Attached world is a random one I used just for crashing, hense the name. 

Description

If you kill wither skeletons that the wither summoned after the wither dies, and the wither skeletons have taken 0 prior damage, killing the wither skeletons will crash the game. 

According to X1Crafts, the factor is the amount of damage the wither skeletons have taken, meaning the wither skeletons cannot take any external damage, and must be at full health.

How to reproduce

1. Find or build a location with a wither trapping chamber of sorts.
2. Summon a wither inside of it
3. Take the wither past half health (Do not kill it) I use /damage @e[type=wither] magic 300, though any damage the wither isn't resistant to works.
4. Teleport the wither skeletons away from the wither, out of range of the explosion. Be sure that nothing damages them. (/tp @e[type=wither_skeleton] @s/whatever coordinate.
5. Kill the wither, any method works.
6. Kill the wither skeletons.
7. Repeat. It sometimes takes tons of repeating

Clip of it happening in 1.21.21

https://www.youtube.com/watch?v=x7Me78au8vw

The exact thing still happens in newer versions.

Clip of me testing in 1.21.31

https://youtu.be/QJiFEYQ1quI

Shows the fact that it doesn't work on the surface. It could also have to do with the amount of time I've spent in the world, as I usually go underground after several above-ground tests. This clip ends suddenly because of the crash. 

Clip from my brothers PC where it crashed higher up in the world. 1.21.31

https://www.youtube.com/watch?v=ihztg5I7Cx0

Not my computer. His game was the only one that crashed. Mine was fine.

Clip of me trying to get it to happen from a fresh launch. 1.21.31

https://youtu.be/aShkfqrnJus

Took two tries to record this clip, as the first time I had a memory leak issue, and the game was lagging and using over 30GB of ram (I've seen it hit 50GB+ before, it's very common for me, should see if theres a report on that already)

Clip of me not having it happen in the current preview. Preview 1.21.50.24

https://youtu.be/V-saJRgVFW8

Edit: Fluke, does happen in the preview.

An unusal occurence. This test is really fun, and also terrible, but will result in a crash almost every time. Oddly enough it didn't crash, and I decided that after 4 (I think it was 4) tests, that it probably doesn't crash anymore. I'm hoping that it doesn't crash anymore once the update is out, because this bug has been stopping me from getting full beacon coverage in my base for a while now.