I tried to use the keyboard option in the iOS Roku remote app in the Prime search and it failed.
I have used it successfully in both the YouTube and Netflix apps on my Roku TV.
REPRODUCTION STEPS:
In the Prime app I use the purple navigation buttons to get to the search screen, tap the keyboard icon at the top of the remote app, type in my keyword tap the ‘done’ button. The Prime TV app does not recognize my keyword and forces the use of the on screen keyboard in their TV app.
AMAZON’S REACTION:
A representative I spoke with at Amazon told me this was due to licensing contracts. I believe this is a bug. I hope reporting it is enough to bring it to your developers’ attention.
Aside from this minor annoyance, I enjoy the iOS remote app for Roku.