In the example below, ‘departments’ is the name of my multiselect field, and if I replace the variable with an option it works fine, so must be an obvious thing i’m doing syntax wise with my curlies.
have you tried site.index.filterBy('departments', page.title)?
otherwise i think the problem is that you would need to run the query resolution twice. first to resolve the {{ page.title }} then again for the outer query. but thats not what the plugin by @rasteiner is doing.