After a few days, I confirm that resizing the "Capture" window (in the vertical direction) solved the problem.
As the widow position and size are stored somewhere and restored when needed, the buttons are still there when you restart OpenCPN.
Now, there is obviously a problem in the UI widget "packing" or organization code.
Probably not a very complicated one for a knowledgeable coder (but I know very little about wxWidgets dev).
This needs to be fixed by running wxFormBuilder and changing the formats....
https://www.cruisersforum.com/forums/f134/weatherfax-plugin-19-60-0-linx-mint-missing-ui-buttons-278918.html#post3817152