The title says it all. This is another one of those "free mount" bugs. Zombified Piglins on Striders suffocate in a block that the Strider brings them to, they die, and: boom! A free Strider for your uses. I recommend improving pathfinding AI for compound mobs to avoid suffocating.
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1.16 pre release?
Can confirm that this is still an issue in 20w51a.
Can confirm in 1.17.1.
Affects 1.21:
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