I've had this issue for a long time now.
As far as I know there are three button commands while using any music players. One press for pause, double press to go to the next song, and triple press to go to the previous song.
But now it somehow works like this:
- One press/tap: Does nothing.
- Two presses/taps: Pause/resume song.
- Three presses/taps: Play the next song on the playlist.
I cannot go to the previous song because of this issue since the triple presses plays the next song instead of the previous one, pressing four times does nothing.
But disabling Assistant didn't fix this issue for me. I'm using Xiaomi Mi 9 with
MIUI Global 12.0.4 and
Android 10 QKQ1.190825.002.
Tested with 2 wireless earphones and 1 wired earphone with two different music players (PowerAmp (paid) & Audify Music Player) and YouTube app behaving the same with videos.