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.
Re: [OS 11.5] Roku Ultra Recordings freezing up when trying to fast forward through commercials
Same problem here. Tried it all. I’ll give it a few days, then return. Bought the Ultra LT to upgrade the Express or whatever they call it, because that one was acting up. Now this one. Was fierce Roku supporter. But no more. I’d rather have Xfinity than this.