Created attachment 153096 [details]
Screen recording of holding down Ctrl at the 0:05 mark for 5 seconds, leading to a time desync (left: Clementine, right: Media Player applet popup).

SUMMARY
The Media Player applet's displayed time and track progress will frequently get out of sync with the actual audio player.

STEPS TO REPRODUCE
1. Have the Media Player applet being visible on the Plasma panel's system icon area and/or assign a keyboard shortcut to calling it.
2. Open Clementine (or any other MPRIS2-capable player like Audacious) and start playing back a list of tracks.
3. Occasionally open the Media Player applet by clicking on the player icon in the system icon area or press the keyboard shortcut assigned to the applet.

OBSERVED RESULT
When the Media Player applet popup is displayed, one of the following cases happens:
a) the displayed time stays at 0:00 and doesn't move, despite music playing
b) the displayed time is out of sync with the audio player but is progressing on its own (either directly starting at 0:00 or at a previous timestamp)
c) the displayed time is actually correct and in sync with audio player

If out of sync, the time will not update to the correct value until the track changes or the position is changed explicitly in the audio player.

Furthermore, holding down a keyboard modifier (e.g. Alt or Ctrl) while the Media Player applet popup is displayed from the system icon area, time progression on the applet will freeze while the key is held down and the displayed progress will get increasingly out of sync as well. See the attached screen recording.
If adding the Media Player applet to the desktop, it is unaffected by the modifier key interrupt but may run out of sync on its own.

EXPECTED RESULT
The progress bar and time label on the applet popup and desktop version should always reflect the current state of the playback correctly.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
KDE Plasma Version: 5.25.5
KDE Frameworks Version: 5.98.0
Qt Version: 5.15.2
Debian GNU/Linux 11

ADDITIONAL INFORMATION
Reproduced with the audio players Clementine and Audacious, on both X11 and Wayland.