Hi bros/sis,
Manage to solve the issue.
Turns out its what one bro mentioned here - CPU memory controller spoil.
Here's what I did for reference:
1) I brought my Asrock X570i M/B to service center to check. With my rams, service guy was able to boot to bios without any issue. Tested with another separate set of his ram and worked fine too. I didnt bring my CPU along as I thought it was either M/B or ram issue. On hindsight I should have brought my CPU along.
2) I bought a new Asus X570i to test. Same issue with my Asrock M/B, able to log into windows with only one slot of ram working. Brought the new M/B to service centre to test and upgrade bios. Confirmed no issue.
3) Brought my CPU & ram to Nigel's shop to test. Both working on his M/B. Able to log into bios. Tested with another of my standby rams too and these worked too.
At this point, every component tested and worked, but put them together issue arise.
4) Brought my whole system with the new Asus X570 M/B, my CPU, ram to Nigel's shop to test. Result is as per point 2.
5) Tested with his CPU chip and was able to log into bios showing both my ram, so Confirmed CPU memory controller spoil.
The underlining issue for this whole perplexing troubleshooting result is that my CPU (although spoil) can work on different M/B that's why its hard to pin point where the fault lies.
So next RMA the CPU then.
Thanks for all the help.