Steps to reproduce:
1. (Optional) Set your Render Distance to a very low value. (Like 2 chunks)
2. Dig a ditch deeeeeep in Netherrack. (Prefferably using an Efficiency V Diamond / Golden Pickaxe.)
3. Go through it, then return.
4. See that the wall was refilled.

This might be due to the thing that chunks don't save again if they were already saved 30 seconds ago, provided that Netherrack breaks quickly (might be as well caused by anything else; since there is also a similar glitch with End Stone) This might make an infinte blocks exploit.