I too am unable to start imac27 2020 with 128 go (4 x own 32gb). It does start and seems to work ok with original 2 Apple 4 go ram in slots 1 and 3 and with 32 gb owc ram in slots 2 and 4.
There was no start activity at all with 4x32gb installed (double checked properly and full in place)
is it possible that it has to initialize with Apple ram installed then change out last two Apple ram with remaining own ram?
Any other ideas as to how to get full 128 go ram working?