I've been having major issues with my iPhone 12 Mini since the 17.3.1 update. Phone randomly shuts off (when the battery is at or near full charge - battery health puts the battery at 99% of new capacity), and sometimes won't reboot unless connected to charger. Uses cellular data only, refusing to use wifi, even when on strong wifi. Outgoing SMS messages are always "failed to send" - progress bar gets to about 90% then fails (incoming messages are no problem, though). Constant use of data roaming (despite turning everything off to try to stop this) goes through several gigs a day, resulting in data plan being exhausted almost immediately. Phone sometimes gets hot, and goes through 90% of battery in 5-10 minutes. Other times battery lasts for a day or more.
At one point it insisted that I had incorrectly entered my password several times & made me wait for 30 minutes. Strangely, the phone had been sitting face down on the coffee table for 2 days...and I'm on facial recognition for opening the phone. On another occasion, it insisted the phone could not be used because there was water in the charging port...again, having been sitting indoors, on the coffee table, high & dry, in low humidity.
There are many other weird issues as well, randomly coming and going.
At this point, it is occasionally usable as a phone, but nothing more.
I sure hope Apple comes up with a fix. My plan is to do a backup & restore to my PC computer using iTunes...if it will work. At this point I don't trust it as I tried this a few days ago, got to 98% of completion, and iTunes suddenly declared that the Apple website it uses was no longer available.