We have the exact same problem with a newly bought Roku Ultra 4k device. After a fast forward or rewind, on many of the apps including Netflix, YouTube, and others, the video freezes, but the audio and subtitles progress. The only way to fix is to restart the device. It might be a bug causing out of memory issue. This should be easy to reproduce.
Besides this, we get occasional HDMI/HDCP errors, but only on Amazon Prime App (again need to restart to fix).
I hope the Roku team can provide an acknowledgment of the issue, and a timeline to fix. This has made the device unusable at times and if not fixed, I'd have to return it and find an alternative streaming device.