Hello. Not OP but having the same problem and wanted to add a response to your suggestion (which is much appreciated!) that, for me, it exits live TV and goes to the "Live TV on the Roku Channel" home screen. From there you can select "Live TV Channel Guide" and will once again get the guide to work the one time only. Once you choose a channel, further use of the guide button yields nothing other than sometimes quickly flashing what's probably the guide for a nanomillisecond but, ultimately, doing nothing.
Just reporting findings so far and will return if I find an answer!
- iPhone 13 (A2482) on iOS 17.3.1
- Roku app version 10.0.3.11287143.5