I had the same problem today.
Created a few forest objects, saved, saved a new file. I noticed the error because I had to open the first save to verify something, then I noticed some FP object were missing, the error said something about the distribution map wasn't loaded or something like that, I selected one of the objects and it was regenerated automatically but with the default distribution map. I applied the correct map save the file and re-open it, but the error was still there...
I tried a few things like saving again from the second saved file (still opened), exporting just a few objects to test and the error still was there.
I had to load one of the faulty saves again, re-assign the maps and pray, and for some reason it worked that time. maybe praying helps. :/
Just hoping this won't happen again, it took a few hours to continue my work.