I do not know what is wrong and still unable to reproduce it. I made another test with the AppImage on Ubuntu 20.04 using a mixture of 80 JPEG and PNG.
I suppose it is possible there is something wrong with loading some incompatible png and jpeg libraries from the system instead of the bundled ones, but the only way I see how is by running Shotcut.app/bin/shotcut directly from the portable instead of one of the launchers. You can try with the flatpak if you want even more isolation between the app and the OS.
ddennedy@Gamerbox-9400:~/Desktop/Shotcut/Shotcut.app$ source source-me
ddennedy@Gamerbox-9400:~/Desktop/Shotcut/Shotcut.app$ ldd lib/libQt5* | grep png
libpng16.so.16 => /home/ddennedy/Desktop/Shotcut/Shotcut.app/lib/libpng16.so.16 (0x00007fa2afb99000)
libpng16.so.16 => /home/ddennedy/Desktop/Shotcut/Shotcut.app/lib/libpng16.so.16 (0x00007f384756a000)
libpng16.so.16 => /home/ddennedy/Desktop/Shotcut/Shotcut.app/lib/libpng16.so.16 (0x00007fd87a326000)
libpng16.so.16 => /home/ddennedy/Desktop/Shotcut/Shotcut.app/lib/libpng16.so.16 (0x00007fc9e2441000)
libpng16.so.16 => /home/ddennedy/Desktop/Shotcut/Shotcut.app/lib/libpng16.so.16 (0x00007f2120a14000)
libpng16.so.16 => /home/ddennedy/Desktop/Shotcut/Shotcut.app/lib/libpng16.so.16 (0x00007f52c9ddf000)
libpng16.so.16 => /home/ddennedy/Desktop/Shotcut/Shotcut.app/lib/libpng16.so.16 (0x00007f5bc28ee000)
libpng16.so.16 => /home/ddennedy/Desktop/Shotcut/Shotcut.app/lib/libpng16.so.16 (0x00007f4f7a866000)
libpng16.so.16 => /home/ddennedy/Desktop/Shotcut/Shotcut.app/lib/libpng16.so.16 (0x00007f202a63c000)
libpng16.so.16 => /home/ddennedy/Desktop/Shotcut/Shotcut.app/lib/libpng16.so.16 (0x00007fc249168000)
libpng16.so.16 => /home/ddennedy/Desktop/Shotcut/Shotcut.app/lib/libpng16.so.16 (0x00007f79babe7000)
ddennedy@Gamerbox-9400:~/Desktop/Shotcut/Shotcut.app$ ldd lib/qt5/imageformats/* | grep jpeg
/home/ddennedy/Desktop/Shotcut/Shotcut.app/lib/qt5/imageformats/libqjpeg.so:
libjpeg.so.8 => /home/ddennedy/Desktop/Shotcut/Shotcut.app/lib/libjpeg.so.8 (0x00007fc17c497000)
Next, I installed Linux Mint 20.2 Cinnamon in a virtual machine, loaded the same images, and exported with no problem.
