The Bug

Armor that can spawn naturally on mobs such as zombies can spawn alongside the special armor with enchantments and trims given by the Ominous Trial Spawner. For example, a zombie can be spawned from an Ominous Trial Spawner wearing an iron chestplate with a trim, and unenchanted golden boots with no trims, causing inconsistencies.

How to Reproduce

Place a command block with a button next to it, containing the following command:

/fill ~6 ~ ~6 ~2 ~ ~2 minecraft:trial_spawner[ominous=true,trial_spawner_state=active]{ominous_config:{spawn_potentials:[{data:{entity:{id:"minecraft:zombie"},equipment:{loot_table:"minecraft:equipment/trial_chamber_melee"}},weight:1}],ticks_between_spawn:20},spawn_data:{entity:{id:"minecraft:zombie"}}}

Set the game difficulty to Hard so that the zombies have a higher chance of spawning with the natural armor:

/difficulty hard

Press the button. Watch the zombies that spawn carefully. In case none of the zombies has the natural armor on, kill all the existing zombies to allow new ones to spawn:

/kill @e[type=minecraft:zombie]

Note how some zombies have armor without trims, such as leather or gold armor, which causes inconsistencies.