If you place down a Trial Spawner and then insert a mob via a mob spawn egg, kill the mobs, the Spawner will not drop Ominous Keys if Ominous mode is enabled. I believe this has to do with the NBT data of the block, but I assume this is a bug because why can't I make a custom Trial Chamber with creative mode I figured that would make sense.

In summary, Trial Spawner when in Ominous mode (Spawners placed by a Player), do NOT drop an Ominous Key despite being in Ominous Mode.