I've installed Ubuntu, installed Mate on it, customized placement of panels. Now every time I reboot, the battery indicator icon isn't shown on the panel until I right-click it, choose "Add to Panel..." and select Battery Charge Monitor. This isn't an ideal work-around, because I want the battery indicator icon to be shown always.
The related question doesn't help, because in my version there aren't options about the icon in the General tab of the settings window.
I'm experiencing the same issue on Linux Mint Mate 19.3. Based on Steve's excellent observation that the battery monitor appears after changing the panel size, I wrote a quick script to run automatically after logging in. You can change it to reflect the specific panel and size you want to change. I called mine panel-fix.sh. My original panel size is 25, so I'm just toggling it to 26 then back again. You'll want to add this to Mate startup applications (just search in the menu) and set a delay. I chose 10 seconds. Works like a charm!
#!/bin/bash
gsettings set org.mate.panel.toplevel:/org/mate/panel/toplevels/bottom/ size 26
gsettings set org.mate.panel.toplevel:/org/mate/panel/toplevels/bottom/ size 25
Hopefully this helps someone else. This was driving me crazy!
Just found this and had a similar problem as a first-time Linux laptop user. In my case, the nature of the disappearance is easily rectified by adjusting the size of the panel up one pixel, and then back down one pixel to what I want. I wish I knew why it was doing that. I wish I didn't have to do that. But it's how I fixed it.
