Compass freezing up in iOS 15.1.1

Okay I have noticed something really weird. I have a brand new iPhone 13 mini with iOS 15.1.1 - I'll be going to Genius Bar anyway for another reason and ask about this, but maybe someone has the answers to this riddle.

So... sometimes the compass was off by 90-180 degrees, and it just refused to right itself while indoors - true, it's a concrete building, so whatever, but... then I went and turned off Privacy > Location Services > System Services > Compass Calibration... and the compass just froze up. Pointing to North, and showing '-- Degrees'.

I've seen an iPhone do this before (showing consistently wrong directions with Compass Calibration turned on, and the compass freezing with it turned off), and its sensors were physically damaged, so I got a bit weirded out.

Apple support ran diagnostics on my phone, and it showed no hardware issue. But I have another iPhone (an iPhone SE 2020 with iOS 15.1) right in front of me right now, with Privacy > Location Services > System Services > Compass Calibration turned off, and its compass is still working.

There's an app called 'sensors toolbox', which gives raw readouts of all the sensors - and the magnetometer seems to be working okay on the iPhone 13... at least as well as on the SE - both seem to be kinda weird to the uninitiated eye, the range of the x axis seems to be between like 110 and 210 uT, while y and z are all in a range around 0 - is this normal? Maybe some built-in magnet skewing the measurements?...

But anyway, the SE has just as skewed raw readouts as the 13 mini, but the compass works on that one.

I'm confused. Also, there used to be a compass calibration screen in older iOS versions, I haven't seen it in I don't know how long... Was it removed from iOS? Is this a software error? Anyone seen something similar?