I'll just simplify the description.
I had clips "A", "B" and "C" to add to the timeline from the playlist but "C" was to be played at 4x. I added "A" to the timeline, I changed the playback speed of "C" to 4x and added it after "A". I realized that I should have added "B" then 4x "C" so I removed (right-click Remove) "C" from the timeline, then added "B" and then changed "C" to 4x and added it.
When I played the timeline, "A" had been shortened significantly even though I hadn't touched it after I added it to the timeline.
So I abandoned the timeline and rebuilt it again.
This same issue occurred in another video I pieced together from clips.
Linux Mint (up to date), Shotcut (up to date)