Further Details: I have tried two different Windows 10 computers with HD video displays 1920x1080 and 1920x1200, and Playing-To one of four different audio devices: 1) HiFi-Cable virtual audio device 2) iFi Audio portable USB DAC 3) Realtek HD Audio device integrated into ASUS PC motherboard 4) PSAudio Network DAC
The first three devices all suffer premature progress bar reset in the same way: Very smooth progress bar advancement through the track, until a sudden reset to zero position at the moment that next-track prebuffering occurs.
Playing-to the 4th device, a streaming network DAC, MC29 does not experience the premature reset problem. Its progress bar advances in a sporadic, jerky manner (sometimes even jumping backwards), eventually making it all the way to the end of the track.
EnglishTiger's comments were helpful confirmation of the problem, and I was able to observe his described instantaneous toggling of the Play/Pause button display at the moment of prebuffering -except for the Network DAC, which toggled the Play/Pause exactly when the next track began to play.