Magnetic mask mattes missing after deleting render files

I did a few tests on FCP 11.2 on Tahoe 26.0.1 on M1 Ultra, and I don't see those behaviors. I created several magnetic masks, analyzed them, rendered to cache, then did File>Delete Generated Library Files..>Delete Render Files>All, and also selected the checkboxes Delete Optimized Media, Delete Proxy Media, and Delete Unused Magnetic Mask Files. The masks were still there. I re-launched FCP and they were still there. Please state your FCP version, MacOS version, year and model of Mac.

There was a previous report of sending an FCP project to Compressor which would produce that error, but you didn't mention Compressor so I'm assuming your case is FCP only. But I tried sending the batch with magnetic masks to Compressor 4.1.1 and it still worked.

If there is any chance you are on FCP 11.0, there were a lot of magnetic mask fixes in 11.1, which would be retained in 11.2.

There have only been a very few reports of this, and unfortunately, the users who reported that never followed up and gave more specifics. I did more testing, doing 'delete unused magnetic masks' many times in various timeline configurations, rendered and non-rendered. I could not reproduce it. Can you give more specifics about your scenario, e.g, do you have multiple magnetic masks on stacked clips, other effects on those clips, or what? Can you reproduce this in another test library, or is this a one-time event that you've never seen before and you cannot reproduce it?

I'd like to see before and after screen shots of the Inspector pane.