So this has been a bug I noticed introduced since some time around late last years. Tested in Shotcut version 19.06.15 and is still present.
Step to reproduced:
- Set video mode to 1080p 60fps
- Take a 1080p 60fps video clip
- Import a 1080p 60fps video clip and make a number of edits to it, such as split cuts, take off 10 seconds of the start of the clip for example and remove a section in the middle.
- View playback f the clip
- Go to Setting > Video Mode and now select 1080p 30fps as the video mode
- View the clip again
The video will have had the split points changes and the timings will no long sync to the original cuts and will play back differently. In short, this bug means that you cannot change the video mode one you have started editing. Shortcut seems to incorrectly handle frame rate changes now in the video. This issue did not original occur in earlier builds, certainly from around 2017.