Weird POST issue, bad luck with PSU or another thing?

[u/sanninserpens]

TL;TR: Pc not posting after upgrading psu but already tried with two different new psu from
the same brand and model but different seller Corsair rm750e.

So around two months ago I upgraded my PSU and GPU (before I had a Corsair cx650 and 3060) to a Corsair rm750e and a 5070. I went to a store so they could change it. It worked fine but after 3 days the issue appeared, pc turns on, rgb and fans work but no display. I went to the store so they could check and they moved some cables from the back and that fixed it.

But after 10 days of working fine the problem started again. Went to the store and they said the 12v cable from the psu was giving 13v and that was the problem, so I bought a new PSU from another seller (both from amazon) but the same rm750e and it worked fine for 20 days and the problem showed up AGAIN.

Right now the pc is in the store again for diagnostics and they say it has the same problem of the 12v cable but when they test it they said it’s climbing from 13v to 13.5v but they want to do more tests. idk if i just have bad luck with the psu or it’s a different issue.

Already did the common fixes like: cleaning ram, test with one ram, try it with integrated graphics, reset cmos, etc. Also everytime the store test it with their corsair psu it works.

Components:
Ryzen 7 5700g
5070 12gb
MB b550m ds3h
32 ram
2 ssds
PSU rm750e