Look, I strongly believe in the hypothesis this is a hardware (antenna) issue. Here's the broad
strokes
What is most damning, is now that the XR FCC
filing are out today, it looks like the antenna on that phone that does not have the 4x4 mimo is an absolute beast with great reception, but more importantly, the XR does not push the power to the antenna to the max like the xs and xs max. The fact that apple would max out the power going to the xs and xs max at the expense of battery life looks like they caught the antenna problem late in the manufacture cycle, tweaked the firmware to do what they could (although adding power has limited impacts on antenna gain, but some for sure.) Also compare how much better the xr is on battery life. Antenna power is a constant factor that can't be shut off, so it looks like a the XR is benefiting from less power to the antenna as well as other factors like screen downgrade. Now Apple is in between a rock and a hard place.
I'm using AT&T window to return my SECOND xs max and go back to my 8+ which I was using for my side by side tests. Likely pre-ordering XR, unless xs max is magically fixed somehow.
If I'm wrong, and I really hope I am because I'm sure this is as frustrating to you as it was to me going through 2 xs max devices with same issues and seeing the problem in side by side comparison with 8+; then I honestly believe you're in for a long and bad time with this device. Apple would have no choice but to slow play this problem as long as possible if it's a hardware issue, and we'll know if they go public about that because they will submit new FCC filings about the antenna if they change it for a replacement. I'm not a hardware expert, but if they had a way to disable the 4x4mimo part of the antenna, that may do the trick, but I'm pretty certain the antennas are powered together as a unit, not individually.
Oh, and just because I don't see anyone saying this clearly: you can't detect the problem with great reception, the problem exists in the margins where you don't have a clear and super strong signal to begin with. It's entirely possible that you have a generally 4 bar daily life, but I bet if you found yourself in a 2 bar zone and did a side by side with a different phone, you'd be hard pressed not to see the problem. Also, just go into airplane mode and test as you get further away from the router wifi, side by side with a diff device. That shows the prob too.