Not sure whether if this is a bug but I think the raiders should only spawn in specific blocks like Stone, Grass Block, Dirt, etc in earlier versions, but now they may spawn on Bedrock.
Bedrock should be used to disable a lot of generating of features, events, etc, and should be able to disable raiders spawning as well.
Feature/Change request, not a bug. Raiders are allowed to spawn on almost any block: full blocks, upside down slabs/stairs, soul sand and blue/packed ice; bedrock is no different than concrete here.