After the update, Magnetic Mask in Final Cut Pro started working poorly.

There have been a few other reports here about vertical lines appearing. The more people that submit detailed feedback, the more likely it will be fixed quickly. I believe the fantastic joema is also submitting a formal bug report.

Send Apple your feedback via Final Cut Pro->Provide Final Cut Pro Feedback... or Feedback - Final Cut Pro - Apple

This is a known problem. I filed a bug on this yesterday. I don't know what the timeframe is for a fix, however it's a fairly serious issue so I optimistically expect that in the next minor release, whenever that is.

I am having the same problem and filed feedback a few days ago. It might get fixed someday, but, you know Apple on their own timeframe and unresponsive. I hope they at least get the feedback in a timely fashion.

At last year's FCP Summit, the FCP dev team told us they definitely read all feedback. For showstopper bugs like this I think the FCP team has been good about updates. I bet we see something within the next 4 weeks. It could be complicated because it might be an underlying macOS Tahoe issue.

Thank you Joe for all your debugging efforts and for filing the bug report with Apple. Hopefully a fix will be released soon.

There have been multiple reports of this issue. Until the bug is fixed, your only option will be to work with older versions of FCP.

●Is there a bug with the magnetic mask in FCP11.2?

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Be sure to back up your data with Time Machine before updating. Rename the old version of FCP or compress and save the app.