What does negative skill progress mean?

Skills above level 10 degrade over time -- the higher the value, the faster the decay. If you have a pawn who focuses on one skill you can still approach level 20, but if you want to keep them there, they have to continue to make use of a skill.

Previously, as soon as dropped below 0, the pawn decreased in skill level. A recent patch changed this behavior to what you observe here; a "grace period" of sorts exists, so that the Pawn doesn't drop in level immediately upon hitting skill XP 0.

So in this case, the last time your pawn was mining, they hit mining level 13. It started dropping once they finished mining, slowing falling to negative values, but not enough to break through the "grace period" and revert down to level 12.

In short, perfectly normal, albeit non-obvious (especially if you didn't notice the change in the patch notes).