Hi Spencer,
There are two operating system limitations underpinning this behaviour.
The current programming interfaces in macOS are implemented in such a way that even one non-secure screen disable the ability to play protected content on all the system.
Apple mentioned to us that this is "expected behaviour" but you can raise an enhancement bug on bugreporter.apple.com to suggest resolving this limitation, as we've done.
More importantly, there is currently no application interface to add secure third party displays to a system. Once such an interface was available we could enable playback of secure content. We have built this capability in our products and as you can see here we are DCP adopters:
https://www.digital-cp.com/licensee-list
We'd encourage you to raise a second enhancement request to allow third party screens to play back secure content, once a secure pixel pipeline has been established to the display of course.
Best regards,
Carlo