Well before I took that step I tried one of the simpler things I’d thus far omitted, airplane mode on & off - resolved! For now anyway, but there’s obviously something underneath going on, whether it be software or, hardware related.
For what it’s worth my current set up is dual sim, with EE UK as Primary (eSIM) & DTAC Thailand (roaming on O2) as secondary (physical sim). Other than when I first bought it & had to use data from the DTAC sim to re authorise a couple of apps, I’ve used EE exclusively for mobile data.
Around here, the EE signal isn’t the best indoors (4G & 5G) & on occasion, I have known it to still switch to 3G. So, on the rare occasion this happens, it may be that the phone is having trouble switching back. I suppose this could be coincidental with 15.2 & actually be hardware related, as was suggested earlier in the thread.
My wife has the identical set up on her 13 Pro Max & hasn’t experienced this issue. She’s still on 15.1, where I’ve suggested she remain, at least until I can figure out what’s going on.