Can you please try to use mainwp_request_update_premium_plugins filter instead of mainwp_detect_premium_plugins_update? Please let me know how that goes.
I made the change. The updates show in the MainWP dashboard now, but not until I go into the child site’s Updates tab and then do a sync on the Main WP Dashboard.
I have seen this behavior earlier, but this is something that is not related to MainWP.
Basically, plugins in the issue use custom update mechanisms and it looks that the process of checking for available updates gets triggered only after checking the Updates age.
This is something that you should check with the plugin author and see if there is something they can do to improve or workaround this problem.