A response from admin@gpxplanner.app suggested the on and off operation yesterday could be due to connectivity problems or deployments during active development.
Whatever the reasons for the problems yesterday, it is working properly for me this morning with consistent mode selection and the subscrition warning appearing whenever I try to save. The three bars icon now brings out the menu and it is now furling away to the left during route editing which it was not doing last night when the three bars icon would sometime work as a toggle and sometimes not work.
I've yet to find any way to modify routing to use as the crow flies or to include paths and tracks but the interface is pleasantly simple to use with the same easy ability to isolate a section for re-routing that is a good feature of GPX editor.
The maps on offer include OS 1:25,000 and IGN for France. I've not had access to these before except on Viewranger on my phones so I see GPX Planner as a very promissing new player.