This causes the snowman@20h drop to occur but the two snow blocks and the pumpkin used to create the snowman don't disappear(thus allowing the infinite creation of snowmen if you pick them back up and keep placing them.) I'm guessing the event is being canceled by otherdrops.

SNOWMAN:
    - action: MOBSPAWN
      drop: SNOWMAN@20h
      spawnedby: BUILD_SNOWMAN

This bug still exists and happens to Iron Golems and Withers too sadly. Its become a bit of an annoyance because all of our mobs are replaced with custom spawns from OtherDrops to give them different HP values.