I seen this normally stone block be changed into gravel. I think it could also become ore/ect.
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Spotted in 24w12a. A spider trial spawner had tuff around it instead of stone.
Seed: 443897344407718545
/execute in minecraft:overworld run tp @s -244 -19 1135 0 20
This can also happen with the moss blocks around a slime trial spawner, particularly if the trial chambers is in a lush cave biome. At the coordinates below are several slime trial spawners with, some of the moss blocks replaced by clay blocks or water sources (Not sure if this requires a separate ticket, but I'm including the info here just in case).
Seed: 443897344407718545 (same as above)
/execute in minecraft:overworld run tp @s -837.0 -20 191 180 0

Found another instance in 1.20.5 Release Candidate 1. Two of the stones around the spider trial spawner are replaced by a redstone ore and granite, respectively.
Seed: 1
Coordinates: /execute in minecraft:overworld run tp @s 311.52 -3.00 293.91 1945.02 32.40

Still present in 24w19b. Two examples of tuff replacing some stone around a spider & cave spider trial spawner, respectively:
Seed: 443897344407718545
/execute in minecraft:overworld run tp @s -1529 -25 1189 0 90
/execute in minecraft:overworld run tp @s -749 -15 1161 0 90

Still present in 1.21 pre-release 1.

Confirmed in 1.21.3
Please provide a seed and coordinates.