I had Bluetooth issues as well. Although my Sony and Bose headphones would connect with Bluetooth audio, the Sony Music Center and Headphones apps as well as the Bose Music and Connect apps would not longer recognize the devices. I could remove and add the devices again to the phone through the apps but then when the apps tried to "talk" to the headphones, the apps failed saying no device detected. Same story with my Amazon echo auto car device where Bluetooth audio would work but the Alexa app built in or on the phone couldn't work together.
This morning I updated to 15.4 (19E5235a) and now the bug is gone and the apps are happy. So are my kids since they can control the music in the truck again 🤨
iPhone 12 Pro Max MG9F3LL/A