Documents or Desktop not Staying in Favorites in Finder Mac Whenever I add the documents or desktop folder to favorites on macOS Sierra, it stays there for some time but then disappears. Is it a problem with iCloud Drive?

Update: It still occurs in macOS High Sierra. I tried booting into safe mode, resetting PRAM and SMC, resetting finder preferences, but it still doesn't fix the problem.

You can see it here:
https://youtu.be/TJubgY3tUKM
Im not sure what is causing this, but you could try force quitting Finder. Go to the Apple icon in the top left corner and select the Force Quit option. A menu will pop up and you can click on Finder to restart it. Also, if you open the Finder preferences you should be able to select which folders appear in the side bar.