My S21 5G (11 months old) has had 3 repairs now for this issue. First one at a service center and the last 2 were doorstep repairs.
From what I understand virtually everything has been replaced and yet the problem persists.
When it came back from the first repair the issue was immediately present. Even though the motherboard and charging port were replaced. And they had failed to glue the rear panel on straight! Shocking quality control!
The second doorstep repair was much better. The technician was appalled by the state of the rear cover. He replaced the whole outside shell including the screen as that's where the antenna is apparently. It worked for 3 days and then issue returned.
Third repair much the same, replaced some other parts, not sure which ones. Again, worked for about 3 days and has now failed again.
As a last resort the sim was replaced as the technician thought it was a bit scratched. Yeah, probably because it's been in and out of phones while all this has been going on!
Again, no difference, issue persists.
I don't believe Samsung really know what's wrong with this phone and I have lost all confidence in their ability to repair it. I can't fault the repair process, especially the doorstep one provided. It's just the lack of a lasting repair that's disappointing.
I'm now looking for a brand new replacement from them.
Not at all what you expect from a top tier phone.