Export results in audio-only, after I added a second audio track

I’m really flummoxed. I imported 4 videos and pieced one part of each together to form a new video. I wanted to replace the audio track with one from an MP3 file, but when I detached the audio, it looked like the detached audio only applied to one segment. I wasn’t sure what to do, so I figured I’d export the video, and then work with the audio in the new .mp4. I did that. I successfully detached and replaced the audio. Then I added a second audio track and pasted another .mp3 into that. When I played the video, it was just what I wanted. Then I exported it. I did not change any export settings, but what I got was just an .mp3 audio file. I tried again and got the same thing. I have no idea why this is happening, and when I read a couple of forum articles pertaining to the same issue, it didn’t seem to relate to what I did. I tried repeating the process, combining three videos but not editing them. I replaced the first audio track and added a second. It exported fine. So it boils down to 2 questions:

  1. What could have happened to cause my original project to result in audio-only exports? And can I fix that project?

  2. If I can’t fix the original project, is there a way for me to copy at least the video portion I edited together and use it in a new project (which presumably will export correctly). Or do I have to start from scratch?

(In case I confused you with “detached audio only applied to one segment,” I’ve attached a picture of what I meant. But that’s a secondary issue, and probably me just not understanding how Shotcut works.)

The default is mp4 with video and audio. You may have accidentally clicked the audio/mp3 export preset. When something like this happens, click Reset at the bottom of the Export panel and try again, or restart the application.