[Kdenlive 18.08.2 - version in Ubuntu 18.04 repo]
I created a Kdenlive project with the HD 1080p 30fps profile. I then added clips that were in 2 formats: 640×480 (4:3) 30fps and 1920×1080 29.97fps. This was rendering fine—4:3 segments letterboxed and 16:9 taking the full frame.

After many hours work, I had some 4:3 clips that I wanted to reverse and speed up. Using the clip reverse is OK, but you can’t add any other effects, so I decided to render the reversed clip separately and add it back. I created a new project and a new profile, matching the source resolution (640×480 @ 30fps), added the source clip, reversed it, added that to the timeline and rendered to a new clip.

After reopening my original project and adding the rendered reversed clip, the old project seems to have become polluted by this new profile, even though I never selected it in that project! The 4:3 segments display full frame and the 16:9 segments are now reduced and letterboxed. Every title I open warns me that it was created for a different frame size. The titlebar still says HD 1080p 30fps though.

On opening the XML (.kdenlive) file for the project the profile listed at the top was the newly-created 4:3 one. I had a copy of an early version of the project, so I backed up the latest one, then copied the profile line from the older backup. This made no difference. Though there is nowhere I can see in the GUI that refers to this other profile, it still seems to be active somehow.

Is there a setting somewhere I’ve missed that could do something this strange? I’ve tried opening the project in (flatpak) version 18.12.1, but it’s the same (except for the added problem that it can’t see the fonts I used in the titles).

I’m currently wondering if it’s worth putting in the hours to recreate this (considering it could just happen again) or moving to another program.

Just a guess. Rename/delete kdenliverc and the session folder, see here: https://community.kde.org/Kdenlive/Configuration.