So we contacted Roku themselves about this and this seems to be an issue on most devices:
You'll find that this type of functionality is not supported by the following devices:
Otherwise, you'll have to wait for images to load before beginning video playback.
Don't take that as a gospel - i asked around and the tech.person answering meant "not recommended because of performance concerns" (with such-and-such devices) which changed to "not supported" by whomever communicated it.
It's possible there is a bug that manifests only on low-end devices but can be addressed. Can you write some "minimal example" code that demonstrates this, to use as a basis for an issue? Also if you can report firmware#, player model#