I’m not sure if this is a wait-and-see, but Alexa+ does not support Roku devices (or probably any Video & TV devices) that come from Home Assistant. It said I needed to setup the device in Settings => TV & Video. I got as far as it asking me to login to my Roku account, but I stopped. I tried removing and re-adding. I unexposed then re-exposed the devices to Alexa under Voice Assistants. I could even see the devices got removed and re-added in the Alexa app. Still no joy. After reverting back to “old skool” Alexa, I was immediately able to control my Roku again.
I have my Roku Ultra devices added in Home Assistant through the Roku integration and exposed to the Alexa integration through NabuCasa cloud.