Master Page Spreads GONE

[Pisana]

I set up a number of master page spreads.

One day, I open the file and all the master spreads are broken up into single pages. When I try to merge them together (for example, putting Master A and Master B together in a spread like they were originally), the program puts out a warning that I can do it but ALL PAGES IN THE DOCUMENT BASED ON MASTER B will be erased. 

Why did the master page spreads suddenly break up into single pages?

AND

How do I get the master pages back into break layouts?

[Pauls]

Have you somehow switched the document layout from 'Facing pages to 'Single page' inadvertedly ?

[Pisana]

These are the settings:

 

And changing the spread setup from page to spread makes no difference with the way the Master Pages are showing up:

 

 

This is quite a frustrating quirk. Is it a bug in the software or is there a way of changing it back to master pages showing up as spreads?