Query for multiple terms of a custom taxonomy on CPT gives 404

I have a custom post type with several custom taxonomies. I’ve got another plugin set up to filter the list of posts with checkboxes. This all works fine when I check a single term from each taxonomy. But when I select multiple terms from one taxonomy, the page 404s. It should just list all the posts which fit either of the terms (or even if there’s an and/or mistake, it should just show an empty list instead surely?). What’s going wrong here?

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