What seems to have escaped many, including HT, is that for Mando they actually created a new stormtrooper helmet that is closer to the original ANH helmets than the "new Disney OT" stormtrooper helmet that was created for Rogue One.
It's hard to say given the limited screencaps available so far, but it possibly appears to be a rework of the RO helmet (as opposed to simply using an OT replica helmet) where details like the the goggle shape have been sharpened/reworked, wider/prominent "teeth" and the jawline "vent slots" redone into a more OT look.
I noticed it in the trailer but I still haven't seen Mando in full resolution (just one youtube reviewer screens) so wasn't 100% sure.
HT is basically reissuing their RO stormie (which isn't even all that accurate to how the helmets look in RO anyway) which is incorrect for the stormies in the Mando room shootout scene.
To add to confusion, the RO style helmet DOES actually appear in Mando - but they are the "junk" background helmets (like on the pikes and buried in the sand) - which I assume was a deliberate "FU" to the revised Disney version. There's no other reason why they wouldn't have just simply made a half dozen more of the unique/new Mando style helmets. It obviously could be budget/time/availability, but we're probably only talking a few hundred bucks each.
Here is OT, new Mando style, RO and background "RO style" Mando helmets: