My "Meta Stack Overflow" site details are not listed in the API, when I use meta_site;main_site in the types parameter. See:

        /2.2/users/2855348/associated?types=meta_site;main_site

But, if I filter for meta sites only (types=meta_site), I can see the "Meta Stack Overflow" details in the API result:

        /2.2/users/2855348/associated?types=meta_site

Is this a bug or am I missing something?