To give an update on this, It's something I've suspected. If the generated block is moved by a piston the block gets "marked" as placed by a player thus gaining 0 EXP when mined afterwards. I've tested it by mining the block that keeps getting generated in my stone gen and got EXP. but any other block after that, none. I know for a fact that this is something In McMMO itself. It's the plugin's doing. hope that helps.
~ @ToonMichaela
I completely forgot about this thread and didn't provide any updates for this issue. Yes, it's built into mcmmo. The Blast pickaxe seems to be behaving the same way in order to prevent power leveling of the skill line.
You can move this thread to resolve/denied as this is not a meep issue. Sorry for bringing this up without doing some testings first.