This one's a doozy....
Right before my eyes! I was me deleting one of those spacers on the sidebar which I could tell is what triggered it. It's at 04:03 and immediately the 3 task lists below the spacer just disappear.
It should've put them into the "Smart Lists" drop-down group, if anything, but it dissappeard. It's okay in this case but it makes me worry about the future, should this happen again and I lose more important stuff.