iseawitch
Rumble likely did not say to "check with Roku". They should know they are responsible for maintaining their channel/app and have been addressing some bugs with one of their prior channel/app updates.
Rumble (version 9.12 build 46) is working pretty good as of late. If you do not have this version, try performing a manual System update on your RokuTV. (Settings/System/Software Update).
As for the Rewind/FF issues, each channel/app performs this differently (depending on how the developer chose to code). For FF and Rewind, do not hold the button down. For Rumble, you press once, (for 1x speed, approx 30 seconds), or twice (for 2x speed, or approx 60 seconds), or 3 times (for 3x speed, approx 90 seconds). You can also use the Arrow keys to slightly advance/rewind as well. (10 second increments).
I haven't run into your Pause issue (starting off in different place), but if an option, try pressing the "Live" button if present.
If you continue to run into issues and the steps above do not seem to help, you can contact Rumble support directly at support@rumble.com
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Alternatively, you can also install RumbleTV on your RokuTV. This is an older, deprecated Rumble version (version 8.13) but still seems to works. I use it occasionally as a backup when the main Rumble channel/app version doesn't work or is broken via a Rumble or Roku OS update. (kind of like the simple interface) (https://channelstore.roku.com/details/458ecef80b412327152af7d77b343632/rumble-tv)