Put the summary of the bug you're having here
What I expected to happen was...:
I could mine coal ore quickly.
What actually happened was...:
Whenever an xp-orb was collected while mining, my pickaxe was repaired by the mending, and the mining progress of the coal ore I was mining while my pickaxe was being repaired was set to 0, which causes the mining progress to become slower.
Steps to Reproduce:
1. Getting mending on a pickaxe.
2. Mine multiple coal, redstone, lapis lazuli, emerald or diamond ores.
3. Your pickaxe get automatically repaired by the mending
4. The block you're currently mining, will not be mined, and the progress of mining the block is set to 0. This will get pretty annoying when you mine a lot of ores.