I have a Samsung a21s and I also use a Polaroid pa80 watch. As soon as i lock my phone the Bluetooth connectivity goes off and i have to go into Bluetooth or the app to connect it again. I did clear the cache and i did reset die Bluetooth and network settings.
I went to device care and i added the app for the watch to the list of apps that won't be put to sleep. Is there a way I need to add the Bluetooth aswell because I have tried to do the same with Bluetooth and had no luck