Just to tell that something like this happened to me and I don’t know why. I was just editing the same way I use to and when I exported the video it was 1920x1080 but with big black bars all around when opening with Windows Player or VLC.
After I close Shotcut and open the project file, Shotcut offered to repair the file which I let it do. I exported the video again and the black bars disapeared. I made a diff between the file here is and example of what changed before and after being repaired:
Before:
<producer id="producer0" title="Shotcut version 17.03.02" in="00:00:00.000" out="00:10:24.000"> <property name="length">00:10:24.034</property>
After:
<producer id="producer0" title="Shotcut version 17.03.02" in="00:00:00,000" out="00:10:24,000"> <property name="length">00:10:24,034</property>
The difference is with the change of a . (dot) to a , (coma) in all time references in the file. Maybe this isn’t related, I think this already happened in the last project I edited with the 17.03 Shotcut version.
What I remember is that the other time I didn’t exported the video within the same Shotcut session I was in when started the project, it was later that I did it after closed and shuting down the computer.