Here is a gif that shows the bug in action:
https://ibb.co/dKqDdV
Steps to reproduce:
1) Work on media item, cut, edit and position media items according to the needs of the project.
Expected Result
Media items will stay in the position they were in, will retain their length, their playback rate, gain, and envelopes.
Actual Result
Without warning, and without any discernable pattern, hundreds of media items change their length, playback rate, gain and envelopes. Some clips obtain a length of 0, and a playback rate of 0.00000001 (or even peculiar words like "nan"). This is documented in the above gif where the Project Bay shows many media with a length of 0, that also misreport their location within the project file as all being at the same location place as the video file I imported, and when I delete this media item video, all these malfunctioning media items misreport their location to another part of the project.
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Above is a summary, but to provide more context:
1) The project is the audio post production for a feature length film. It is from a Final Cut Pro XML file that was converted into a Reaper project by the program Vordio.
2) I imported a rough cut of the film to edit to.
3) I worked on the project for a week, mainly normalising dialogue. I did not notice anything strange, except what I thought was some drift between the media items and the video file. This seemed to go away when I asked for a copy of the video file with time code hard encoded into the image.
4) Close to the project deadline, one day, a large number of clips in the project were scattered in the way described above. (Some clips obtain a length of 0, and a playback rate of 0.00000001 (or even peculiar words like "nan").
5) I tried to go back to an earlier save, and backups of the project file, but found that upon loading, these older files also suffered from the same corruption.
6) I attempted to restore the media items to their locations. Some time after completing this task it happened again. This was repeaeted multiple times.
7) One time I observed that it happened when I hit "ctrl+z" - instead of the last action being undone, I watched multiple clips move their locations, change their envelopes etc. I have not been able to reproduce this when using screen capture.
8) I disabled all scripts, but the bug occurred.
9)I tried using a much earlier version of Reaper - v 5.51 but the bug still occured.
10) I thought that somehow the bug related to my nextcloud sync, so I disabled nextcloud but the bug still occurred.
11) It seemed through my experiments that copying media items from one of these corrupted projects would introduce the bug also into a fresh project.
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This is absolutely a show stopping bug. I cannot work on any of my projects right now because I have no confidence that the bug will not destroy everything. My appreciation will be endless if anyone has an idea what is causing this and what the solution is. It cost me of sanity this last week and resulted in a missed deadline for a film festival.