The OP is actually right. A full suit of mail under a full suit of plate, like the ones we currently have in the Master rank arena, is unhistoric and not very logical. There was a transitory period where mail and larger plate parts were worn in conjucntion, like a breastplate over a mail hauberk. But when plate became more refined, the mail layer faded away until all it was left were a few patches on the gambeson/arming doublet covering the most vulnerable areas like the armpits; something like
this. (Even later, plate articulation even on the joints became so good that there was basically no more need for mail).
Don´t make a mistake, mail is actually a quite heavy type of armor, usually heavier than plate. Wearing a full set of both would not only weigh you down extremely, but it would also make a good fit of the later kinds of plate armor pretty much impossible - especially on the legs where the plates have to be tailored very closely to the wearers leg shape. So the extreme disadvantage of the extra weight and the extra hindrance would only be countered by a very minor gain in protection, and 90% of said protection gain could also be achieved with just a few mail patches, which is why that´s how it was done.
On a similar note, the layer of padding underneath the armor also became much less bulky and more closely tailored than the earlier stand-alone gambesons, losing a noticable part of their protective capability to better function as a fundation for the plate. They were less about granting protection and more about making plate more comfortable to wear (and also a bit about shock absorbtion of course). But when you look at the bulky earlier gambesons there is no way you could wear one of these under a later plate armor. Again, especially on the (lower) legs there was pretty much no padding at all, just the greave sitting tightly on the calf.
So, that would mean
- the Master-kind plate armor should not be worn over a thick gambeson
- not over a full chain shirt
- not over thick, padded trousers
- not over mail leggins
and instead, require a specialized arming doublet, ideally with mail patches (and, well basically just regular pants for the legs).
PS: If you want to learn more about armor,
here is a very informative Youtube channel. Granted, this guy´s main armor is from the transitory period and still uses a lot of mail, but he goes a long way to explain development, function and use of armor in general.