Explanation of bug:
When you place grass path block below iron bar it remains as grass path block. But when you place or remove any block surrounding or basically updating the grass path block it gets converted back to dirt.

Steps to reproduce:
1) Place an iron bar one block above ground level
2) Now place grass path block below that iron bar
3) Place any other block next to that grass path block
4) It converts to dirt

Expected Result:
Grass path should be remain as grass path or dirt if its inbuilt feature

Actual Result:
Only while updating the block converts to dirt (or if not a feature remain as grass path)