Ah, the much-loved Bluetooth again 
…
- Have you turned the phone off and on again (real shutdown, not just a reboot)?
- You could try whether pairing works again after deliberately “forgetting” the device in question on the phone:
Settings - Connected devices (Bluetooth, Cast) - Bluetooth - (cogwheel for paired device) - Forget - Somewhat radical (at least it sounds that way, I never needed to do this):
Settings - Network & Internet (Wi-Fi, Mobile, Data Usage, Hotspot) - three-dot menu - Reset Wi-Fi, mobile & Bluetooth - RESET SETTINGS - Settings - Developer options (to activate these, tap on Settings - System - About phone - Build number several times) - Enable “Show Bluetooth devices without names (Bluetooth devices without names (MAC addresses only) will be displayed)”