Exactly - mapthings didn't used to have scale, that's a new feature that's been added. The MT_LOCKONINF scale is set during P_SpawnMobj, which was fine in earlier versions, but now, its scale is set again during P_SpawnMobjFromMapThing, which occurs after P_SpawnMobj.
Yeah, I found this out from a Lua scripter reporting problems trying to set scale by Lua, and went to check the code. I told them to use MapThingSpawn as a workaround, but didn't realise it would break anything in use in vanilla... Incidentally, this also breaks what remains of the old "ERZ3 mode" that shrunk everything except the old Brak Eggman boss.