Android: "Version code has already been used"

I go back to Flutterflow and change these values in App settings -> Mobile deployment:

I then download the AAB file, and try to upload the file to Google Play Console, but I receive the same error.

It sometimes works. But I haven't noticed what it is yet that makes it work. 95% of the time it doesn't work.

Clicking "Deploy to Play Store" results in the following build error.

I suspect this is possibly a consequence of the version code not updating. But I'm not sure.