This has been happening to me recently. I am using a Piano from the Roland JV-1080 vsti plugin. Closed the project. Re-opened it at a later date and it is no longer the Piano sound I used but some other random sound. I changed it back and it works fine until I close and open the project again. Is there some trick to making the patch stay permanently? Also, the times it doesn't change instruments, a reverb, chorus or delay that I turned off originally returns the next time I open the project. This seems to happen with my Korg M1 vsti so the issue isn't exclusive to Roland. I don't seem to have a problem if I use Cakewalk's instruments or some others. It could possibly be the 3rd party plugins. Has anyone else experienced this?

Yes, with one single (32 bit, no longer supported) VSTi.  It always reverted to patch 1 (or 0, depending on where you start counting), even though the name in its internal list and in the VST name drop-down was correct.
 
My solution was to select the patch I wanted, export it as an fxp file, change to patch 1 (0) and load the fxp file into that slot.
 
Works a charm (as long as I remember!).