You start to damage a block (anything, ores, bricks, whatever) so that it's almost destroyed/mined.
You wait some time (let's say 15 seconds).
You damage the same block and see that it hasn't been healed at all and gets destroyed within 1-2 hits.
Then, again, you start to damage a block and pause.
This time, though, you damage another block while waiting.
You damage the first block again and see that is HAS been healed.
At least, that's how it is for me on MC 1.12.2 (and Multi Mine for 1.12.1)
I'm using Forge 126.96.36.19901 in case it matters.
I am so sorry, i completely forgot about you and this.
- fixes bug where block regeneration would never affect the block a player mined last
Yes, 1.12.1 and 1.12.2 have the same obfuscation and so mods work on both