When the MainWP Child plugin is enabled on a given site, the display of the available WP themes is reduced to a single theme - the one that happens to be in use at the time. So, it is no longer possible to select an alternate theme from this dashboard while the plugin is enabled. Disabling the Child plugin returns the dashboard to normal, revealing all themes installed - active and inactive.
I’m referring to the WP Admin area (Dashboard/Appearance/Themes) of the child site. To be honest, I haven’t checked the MainWP Dashboard for similar behaviour, if any.
I can confirm that the first option/toggle (Disable theme switching) is enabled. I’ll disable it and report the result here.
Thanks for the quick response!