Update: it's working properly, I just had a 20 villagers long streak of bad luck
In the new villager trading experiments, expert level taiga cartographers are supposed to exchange all types of diamond armor, but after verifying in over 30 villagers, they only exchange boots for leggings and leggings for chestplates, missing the trades of helmets for boots and chestplates for helmets.
Cannot reproduce, cartographers do not trade diamond armor, you must've meant armorers.
Still, after trying to get a few armorers in a taiga biome, I was successfully able to get all 4 diamond armor trades. You must've been very unlucky, or this was an issue only in older releases.
I ran my tests in 1.20.51.