This thought just occurred to me as I glanced over a notification that came in on another thread ("...there does not seem to be a way of seeing exactly which layout is in action for a page, even by inspecting the code." Source)
What about a new little icon in the admin menu that either links to a separate page or has a little dropdown info pane with details about the current page you are viewing?

For example, all in one place, info (and potentially links to edit/adjust) about the:
- Layout used on this page
- Theme
- Content type
- etc.
That may be overkill but thought I'd throw it out there to see what people thought.
Recent comments
Wow, many thanks for keeping on digging. You finally found it! Yes, now I can reproduce the problem, as soon as I switch to "Use map defined setting" in the view's zoom setting section...
Disappearing administration bar. Is this a Leaflet or a Backdrop bug?
Thank you for these clarifications. In a few days I'll tell you more...
Create a new theme - Starter theme
I did a bit more testing again using a new content type. It seems the problem with no sane default map zoom level is with all the maps from Leaflet More Maps (I did not try all of them, but...
Disappearing administration bar. Is this a Leaflet or a Backdrop bug?