The bug

While performing the reproduction steps for MC-123696 the game logged the following error multiple times:

Unknown tag: aew@1c69c647

(aew being net.minecraft.tags.SetTag, Mojang name)

Logger name: cwa (net.minecraft.world.level.levelgen.structure.PoolElementStructurePiece, Mojang name)
Logging thread: Server thread

I do not know how to reproduce this. The same error message is logged when reproducing MC-202186, therefore these bugs might be related.