Exported Lossless Video - Output File Was Lossy
At least using Shotcut, you can get lossless output at minimum file size using the FFV1 codec.


In the export window, choose the lossless/FFV1 preset.
Click the Advanced button at the bottom.
Go to the Codec tab, and set the GOP to 250 frames.
Go to the Other tab, delete everything there and type the following:

coder=1
context=1
pix_fmt=bgr0
mlt_image_format=rgb24



Source: https://forum.shotcut.org/t/lossless-annoyances/18344/2