In my opinion this issue could be related with older phones because they have older Bluetooth hardware...and Apple made a software which uses some new bluetooth features with iPhone SE/ 6s is not handling correctly.
It is possible that they added some new features under the hood and our phones does not handle them as they should.
Our main devices are iPhone SE / 6S ( the same hardware). One person reported problem with 11 Pro, but i think this could be an exception which proves the rule :)
Those new feature could be related with better security, or power management.
I remember that Apple always has problems with Bluetooth stuff after updates. In the past i was struggling with Apple to fix the connectivity with Microsoft mouse...and they did not fix it. After update to MacOS Mojave the mouse stopped to work, but in High Sierra everyting was fine. Strange coincidence that this problem could be related with Bluetooth.
iOS 13 and WatchOS 6 are probably the most buggy operating systems from years.
ps. I am still waiting for reply from engineers who grabbed logs from iPhone and Watch.