If default siteaccess uses URI type matching, URL alias generated by passing default siteaccess explicitly will be different than when generated without passing it explicitly. Since in both cases the URL alias is generated for the same siteaccess, the result should be the same:

Example:

{{ path('ibexa.url.alias', {'contentId': content.id, 'siteaccess': 'default'}) }}

generates:

/default/some-alias

But:

{{ path('ibexa.url.alias', {'contentId': content.id}) }}

generates:

/some-alias