Reproduction steps

1. Open this context in edit mode. Observe that the BG layer thumbnail for Ortofoto Regione Piemonte is visible on the BG selector plugin. The service was imported when the context was created.
   

2. Open the same context in view mode, observe that the thumbnail is not there, and there is the warning message Preview Not Available.
   

3. Save the context as map, obeserve that on the created map the BG thumbnail becomes visible again
   

The expected behaviour would be to see visible the BG layer thumbnail in the BG selector in the context in edit view, point 2.