I will now switch the two Epyc 7551 in this board and test. Maybe they are not seated well still? As after reseating the detected DIMMs changed from C, D, G to C, D, G, H in P1 and from C, D, G to just D, G in P2.
After that I think I should remove the same Epyc 7551 from one of my Asrock Rack...