No network Enabled Themes available for Sub-sites

Hello, I searched but couldn't find anyone else describing this issue. It is not specific to any one theme and I am uncertain if it is related to a specific plugin – most likely Pro Sites – so feel free to move this topic. Thanks in advance for any help troubleshooting...

The only theme showing in the Appearance admin tab of any subsites on our dev install is the currently active theme. There are no other themes to choose from. We do have Pro Sites running and the Premium Themes module activated, but deactivating the module and or the plugin made no difference.

Symptoms include:

The Themes admin page on every sub-site reports: "You only have one theme enabled for this site right now. Visit the Network Admin to enable or install more themes." The one enabled theme shown on each site is different, the currently active theme.

Attempting to enable themes via /wp-admin/network/site-themes.php results in "No Themes Found." even though 10± themes are not network enabled.

Pro Sites -> Premium Themes admin page lists no themes as shown in the attached screenshot even though 10± themes are not network enabled.

Super Admin Themes tab lists 120 themes, all but 10± are network enabled.

Editing a specific site via the Themes tab at /wp-admin/network/site-themes.php?id=x results in "No Themes Found." even though 10± themes are not network enabled.

All themes are available for the primary site (id:1)!

Issue is identical for existing and newly created site.

Attempted fixes:

Disabling and enabling themes made no difference.

Unchecking Pro Sites Premium Themes module and saving changes made no difference.

Manually extending Pro Site level to access Premiun Themes did nothing.

Deactivating and reactivating Pro Sites and other plugins? Nope.

So... What else might cause network enabled themes to not show up for all sub-sites?

For the record:

We are using themes from Farms 133 Theme Pack. I do not know how long this has been going on. I discovered the issue while testing site functionality after implementing some mod_security tweaks.