If you're holding an item in your off-hand (left) and not in your main hand, then you're not able to sit up and down a fox you tamed, even though your main hand is empty. This confused me for a while, because I usually keep a torch in my off-hand, just as a habit, and I thought it was a multiplayer problem! Not that big of an issue, but if it could somehow be changed so that only your main hand needs to be empty, that would be great.

You might be happy to learn that I'd already changed off-hand mechanics for the next release so that you don't need both hands empty to control foxes. Soon™.

I still wanted the off-hand to be usable, though, so even with the new mechanics, if you have torches in either hand you'll just be telling your fox to find them. If you want to stop that, you can add minecraft:torch to the block blacklist.

Was coming to report that I couldn't unsit my foxes...guess this is the reason then.