Cases subpanel missing from custom module

Sugar Pro 7.7.0, self-hosted

I have a custom module that has a many-to-many relationship with the Cases module created in Studio.  I can see the relationship listed in both modules' properties, and all details in the relationship are present.  Both modules list the "Subpanel from" as default.  When I look at a Case I can see the subpanel for the custom module.  I cannot see the Cases subpanel in the custom module, however.

I've done a repair for the relationships and a quick rebuild and repair in case that would help, and it hasn't.

I saw another discussion in here related to 7.5 that said to go into the subpanels for the module (a stock module) and deploy the subpanel, but the Cases subpanel is not listed for this custom module, or indeed for any of the stock modules that have many-to-many relationships with Cases.

Is there a way to have a subpanel for Cases appear in any module, stock or custom?  Is this an indication of a problem with the deployment?

I can handle any coding that might need to be done, but I'm not familiar enough with Sugar to know where to start.  Any direction you can provide would be greatly appreciated.

Thank you.