I have noticed this bug using MediaMonkey 22.214.171.1248 on a Windows 10 Home 20H2 machine. This only happens when I cast the player's output to one or more uPNP/DLNA speakers and not when using the internal renderer (playing through laptop speakers). I have two DLNA-compatible speakers and the problem is there regardless of whether I cast to a specific single one or both of them at the same time using Multi-zone casting.
The track progress seek bar does not position the playback at the correct position in the track when clicking on another point on it. Instead, it momentarily seems to travel to the desired point, only to fall back to the point where it was playing before seeking (again momentarily) and finally falls back to the beginning of the song (or at 0:03 - 0:004 depending on if you've got crossfading enabled). Transitioning from the desired position to the previous position and then back to the initial track starting position only takes roughly a second. The problem can be reproduced whether we are using a normal 'slider seek bar' or the 'waveform bar'.
I have included a video to showcase this issue.