I know this is a 6-year-old question, but the problem still occurs in november 2021 :) I am running a fresh Mint 20.2 with Cinnamon.
The script in the other answer fixes it, but you have to run it every time. So for anyone else coming across this, this is my case. I frequently connect and disconnect a second "monitor" (it's a HDMI output connected to a USB capture device on another PC, so my own PC's output becomes a webcam input). I don't want my "audience" to see my panels, open windows, tasks etc., not even temporarily.
So what apparently happens is that as soon as you connect that second monitor, the system decides that that new monitor is my new primary monitor, and the panel immediately moves to that monitor.
So, simply connect the second monitor, then go to display settings, set the normal monitor as primary, apply the settings, then move the panel to the proper monitor, and that worked for me. Hope this helps anyone else, it kept me busy for over an hour :)