Unable to access popup menu from MFC Ribbon Dropdown button

Can you verify your application is fully supported based on the information given here Supported Development Tools | TestComplete Documentation

Also see Desktop Application Testing | TestComplete Documentation - you might need to do some pre-requirement setups first, and Object Identification | TestComplete Documentation. See https://support.smartbear.com/testcomplete/docs/app-testing/desktop/missing-debug-info-message.html 

I suggest you create a new project, and try and get TestComplete to identify individual UI controls in your application first. Switch between enabling and disabling MSAA/UI Automation *, and switch to Advance View, and not Basic View.

Enable or disable this only, and to not add items to this list

This is to ensure and verify TestComplete can support your application.

I've created a MFC application which uses CMFCRibbonBar, and I'm also struggling to capture the sub-menu items, as it keeps getting destroyed when it loses focus while clicking on Object Spy. So I'm not able to query any of the Params.

This is a lot difficult than the WPF application!

Would you be able to open a support ticket via https://support.smartbear.com/testcomplete/message/ and hopefully, their support team should be able to come up with a possible solution.