This has been solved for me in the LXDE forum.
I learn that LXDE is built on openbox, and this F11 behaviour is set in /home/(me)/.config/openbox/lxde-rc.xml. Search for F11 and comment out (or remove) the three lines. (Commenting out requires the form <!-- .... --> for each line - you have to change both ends.)
To get the best of both worlds, change "F11" to "A-F11" instead of removing the item. This allows F11 for applications, and <Alt+F11> gives the openbox "full screen" mode.