Problem: How to filter view and return the related posts with new Many to Many post relationship API
Solution: You can use view's filter wpv_filter_query to filter view and return the related posts.
You can find proposed solution, in this case, with the following reply: https://toolset.com/forums/topic/having-trouble-filtering-and-returning-related-results-based-on-dropdown-menu/#post-901080
Relevant Documentation: https://toolset.com/documentation/customizing-sites-using-php/post-relationships-api/how-to-migrate-your-site-to-new-post-relationships/#full-documentation-of-the-query-argument
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