It’s because the options you’re choosing don’t exist in blueprints/modular/features.yaml
If you’re happy to change the theme directly rather than override it, then you can go in there and see there are options for ‘small’ and ‘standard’ - it’s just a matter of adding some others. Here’s how mine now looks:
small: Small = 4 / 3 / 2 columns
standard: Standard = 3 / 2 / 1 columns
offset-box small: Offset Box small
offset-box standard: Offset Box standard
The keys are the css classes, the values are the text visible in the drop-down. Once you’ve saved the file, go back into the feature, select the appropriate option and save.