https://www.minecraft.net/en-us/article/minecraft-snapshot-21w43a
21w43a states "Chunk's Level.Entities has moved to entities"
I'll fully admit I do not know in what context entity data is still found in terrain mca chunks as entity data was moved out of terrain mca files in 1.17 - However, 1.18-pre5 /entities/r.X.Z.mca chunks still use the tag name "Entities" instead of "entities" which if nothing else is inconsistent and possibly an oversight.
The
entities
entry in region files is only used during world generation. After that the data is moved to the entities/ folder, which isn't affected by this change.