I tried out this fix and it looks to be working for me. 
Your description of the base issue makes a lot of sense. I was able to make the modifications you describe here and build the Handheld AR project. The default asset seems to stick properly to the ground. I’ll do some A/B testing with a 5.2 build of HeldheldAR to see if there is any performance difference, but it looks like you’ve solved it.
thanks so much!
-e