I am playing MC 1.9, in singleplayer without commands. When I break blocks they keep reappearing despite the break animation. My frame rate was low but the problem was still there after I fixed the frame rate. I have tried fixing the frame rate multiple times with no success in fixing the initial problem.
2 Answers
Generally, what is called "block lag" is from the game taxing the hardware being used (your computer in this case) for more RAM. Normally, Minecraft can get away with a fairly small 2 gigabytes of RAM, but with larger modpacks, it's common to increase the assigned RAM limits to 4GB or more.
This isn't related to framerate. This is related to latency and memory issues. I have the same issue when playing myself with a large modpack like Resonant Rise or a resource pack. Note that the blocks usually eventually do disappear, but you might have to mine them again.
The problem could be related to a lack of memory in Java. You could try and assign more memory. However, I'm not sure if this fixes it.