What I expected to happen was...:
I thought the infested blocks would have stayed infested while I fought the silverfish

What actually happened was...:
the infested blocks turned to regular old plain old boring old stone

Steps to Reproduce:
1. Place infested stone blocks in creative
2. Set mobGriefing to false
3. Spawn a silverfish
4. Go into survival and punch the silverfish a few times
5. The infested blocks are now all uninfested