I have exact same problem i.e no matter what, it just won't connect in Stereo mode, both connect in mono mode. Headset is Lennova HT10 which uses APTX. Same headset is able to connect successfully in stereo mode on PC & other devices like One plus 6 & 7. This definitely broke in last last recent update and is very annoying. I have tried rebooting, resetting headset etc but it still won't connect. Asus screwed it big time and needs to fix this asap