In Slicedlime's dump of the vanilla features, the fancy_oak.json claims that it is using blob_foliage_placer as foliage_placer but it is actually using large_oak_foliage_placer.
This error comes from the fact that the LargeOakFoliagePlacer is extending BlobFoliagePlacer (unclear as to why this is the case), while not overriding the getType (yarn mapping) method.
Confirmed
You can see side-to-side the regular fancy_oak (
minecraft:fancy_oak) and the file copied in my test namespace (test:fancy_oak):{{ minecraft:fancy_oak | }}
test:fancy_oak{{}}