Different fields available for different sites when when customising Menu's

I have 2x websites, both running identical versions of the same theme, yet when I try to customise the menu’s I’m met with different field options.

On Site A I am able to add settings to fields such as CSS Classes, Link Relationship and Description, however on Site B those fields are not available.

Neither site has custom customisation settings, and a brand new install of the theme on a clean DB results in the same settings availability as Site B.

I’m a little stumped by this strange behavior, has anyone experienced this it before?

The images below show the fields available when adding a Custom Link (both via the Custom Link option, and the pseudo Home link under Pages). I have however confirmed that this behavior persists when adding things like Categories, Tags and Posts to the menu.

Site A
Site B

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