Yes, it doesn't work for me either. I have 3 APs, 2 from this one and all devices always connect to the old third. Support has not been able to solve it yet.

We have had 2 U6LRs in the house for about 3-4 months.
IPhone 12 and XR so far without problems, with good data rates.
The APs need some time after restarting (adjustment of the channels due to other Wlan's) until they develop the full performance.
Device Version 6.0.21.13673, Network 7.2.92

iPhone 11 works perfectly with 15.5 and also with the latest 15.6 version.
Unfortunately, I cannot say anything about the connection with iPhone 12.