Today made a clean install to a friends redmi note6 pro, 9.5.9 rom. Problem didn't exist. But even dirty install if this problem solves it would be good.
Same problem redmi note6 pro on some bluetooth devices. Sometimes it connects sometimes doesn't. If i make pair from other device it connects. Some devices no sound when connected. I think driver problem about Xiaomi.
I opened bug report on xiaomi forum for picture in picture full screen gestures mod. Please confirm to get xiaomi's attention. Thanks.
http://en.miui.com/thread-5427174-1-1.html