Documents folder keeps disappearing from Finder

This is an issue that has been very irritating.

Lately I’ve noticed that my Documents folder keeps on disappearing from my Finder sidebar. I have no idea what happened - all I know is I woke up one morning and it was gone. The Documents folder was still fine and in my Users folder, however.

Obviously I tried dragging it back to the sidebar - the folder would reappear in the sidebar for about a split second, and then proceed to disappear again.

What on Earth is happening? Why does this only affect my Documents folder? Why doesn’t it want to stay?

No idea why I put my iMac model here. As if it would actually help.