Just wanted to give you guys an update. I got my replacement phone 4 days ago and it seems the issue is fixed.
The headphone jack on my first 2 phones were very loose fitting, the headphone plug would easily move around by itself when walking while in my pocket.
The 3rd phone has a much tighter fitting, once it is plugged it doesn't move, even if you want to move it yourself by rotating, you can feel it takes a lot more force to move it. I think this could be part of the issue for some people.
If you can easily rotate your headphone plug and have this issue, I would see if you can get a replacement that has a snug fit. My first 2 phones were built in September and had the loose fitting port, my 3rd phone was built in October and had the tighter fitting port.
I think Apple are on to this from that perspective. If there are other contact issues (like the metal ring issue) then that doesn't seem to be fixed as of yet.
I am hoping this will be my last replacement for this phone! I will keep you updated if anything happens over time.
For reference, a bunch of friends have phones with Serials starting C3 and my first 2 were also C3 but my 3rd phone starts with DN and is a week 39 (October) phone. My others were both week 36 and my friends have week 36 and week 37 phones.
It would be interesting to see what factory and week the iPhones with this issue come from.