Based on the Troubleshooting in Subscriber Organizations guide, it seems that when a subscriber has granted you access to their Salesforce organization, you should be able to see obfuscated code and debug logs for your own managed package installed in their organization:
When logged in as a user in a subscriber’s org, you have access that the subscriber doesn’t have. You can view the obfuscated code in your Managed - Released packages, view logs that the subscriber can’t see, and initiate ISV Customer Debugger sessions (Source).
However, when the subscriber grants me access to their organization where the Managed (Released) package is installed, the code appears obfuscated in the Developer Console and the debug logs also appear to obfuscate logs originating from the managed package. The user granting access has the System Administrator profile.
Is there something further that needs to be configured in order to see this information from the managed package? Have I misinterpreted the documentation?