I was hesitant on posting this, since I don't have any screenshots, but I might as well-

• Went to take my wolf mining as usual, left him alone for a bit.
• Came back when he still had a little over a whole empty row still, noticed I needed to replace the wheel
• Replaced the wheel, left for a bit
• Came back, and not only was he "just chilling" with six-seven empty slots left, it once again said there was no wheel attached
• Instead of being at three wheels left, which I was before I replaced it the first time, I had two wheels left
• Was able to get him to start mining again after dropping an ore

So basically something glitched out when I was replacing the wheel and I lost a whole wheel due to it.

Same issue:

I was mining and off to another screen. Came back and my wolf is Just Chillin --- but 2 empty spaces slots left. I tried refreshing, and closing the game and restarting; still not mining. As you can see in screenshot, I had plenty of stamina, with pickaxe and wheels equipped.

https://snipboard.io/dHToYa.jpg

Dropped an ore and mining restarted and all 3 empty slots were filled.