The (imho) most plausible explanation I've read somewhere is a tolerance/variance in bores and holes somewhere between the fluid in the line and the fluid in the reservoir. When viscosity changes (either very hot or very cold temperatures) and you pump the lever quickly for several times, your bite point can vary, because the amount or pressure (not sure how to express that properly in English) of fluid in the line changes and equalization sometimes doesn't result quickly enough. Depending on the fluid (some use Putoline 2,5WT fork oil and claim that it solves the "issue") and probably also the degree to which the bore or hole is smaller than it should be for a quicker equalization the bite point will wander a little or a lot. The problem seems to occur more often and in stronger form at the rear brake, which has more fluid (longer line), smaller rotors and gets dragged more. Which makes a fluid/equalization issue more probable.
Closed system hydraulic brakes like the Hope C2 or the Magura HS33 rim brakes can show a similar, but more dramatic, behavior.
If the brakes are not bled very thoroughly you can probably have the above issue and ALSO bite point wandering because of air somewhere in the system at the same time. Also a misaligned caliper and sticky piston, pad wear and worn rotors.
Combine all that and internet message boards...
However: Shimano DID warranty the odd brake. But they warrantied quite a lot of M8000 XTs back from the first batches. I warrantied a M8000 right hand lever and saw a couple of pairs go through my LBS for warranty.
Now I am riding M8020s (same lever, 4pot caliper) and they are almost not showing this behavior.
I bought some M8120s and am curious how these will feel. Mostly for the fatter lever blade and the stiffer clamp.
My personal voodoo, apart from not bleeding my brakes while drunk, is to lube the lever piston with a q-tip and some mineral oil. I swear it solves the issue!
As the mineral oil doesn't damage my paint and is not THAT poisonous I don't mind bleeding my Shimano brakes. With the little plastic bucket it is so easy...