Hello,
I guess you are using P3dv4.4?
I have noticed this behaviour in v4.4 some time ago, too.
It seems to be P3d related.....there‘s already a thread with this issue on the P3d support forums, but isn‘t fix yet, I think. For some reason objects seem to disapear in v4.4.
After loading Misty‘s Place into the sim, I just load it a second Time via the P3d menubar.......after this second loading, the trees show permanentes at Misty‘s Place.