I'll try using reset controllers to zero; all midi echo is turned off (that's part of my solution to other midi issues, also never use VSTI synth midi output, and never use "omni" for anything anywhere). 

williamcopper
all midi echo is turned off 

Okay, just wanted to make sure you were aware of the preferences setting. 

williamcopper
Shifted ahead 100 bars, started playback, and the pan was forgotten.
 
...zero controllers when play stops set no.
 

 
If zero controllers is disabled, the soft synth's pan position should have persisted, regardless of whether searchback worked or not. There would have been no reason for the pan to re-center, unless possibly you also have the MIDI track pan enabled. If you want to use controller events, you should make sure the track widget is disabled.
 
I'd check that, and if problems persist, try directing that track to a different synth.