Hello, my 2nd tier team has performed a few tests and concluded that the problem here lies with the theme since the same issue can be seen if Toolset is not active. You can see and share their comments here with the theme developers:
https://coda.io/d/P2-Replies_dL0rKbi6lvP/P2-5494_suin4#_luXt2
Issue: the category/films/ archive is missing CPT film posts, it only shows standard Posts (if you add the category films to some posts for testing).
After quite extensive debugging I confirmed that it has nothing to do with Toolset, the issue arises because of the theme.
If you add and activate Query Monitor and then load the category/films/ archive you will see there is a single unrelated query from Types (checking the existence of relationship database tables).
see query-monitor.png
Most of the queries on the page are originated by the theme.
We can demonstrate that the problem still occurs without Types.
We need to register the Film CPT using register_post_type, and then we also need to use pre_get_posts to modify the Main Query on the category archives so that they include the film post type (by default they will only return Posts).
Doing that (with Types inactive) we get exactly the same result, namely that the category/films/ archive omits the film posts, while the archives for other terms such as festivals work correctly (try adding the category film to some festivals posts).
I uploaded the theme functions.php edited to include the required code: https://drive.google.com/file/d/1UJlKgWu4ub_NILQqPeRHY5xvPODmFEV9/view?usp=sharing
That file also includes a callback hooked into the posts_results filter, which shows the posts returned by the Main Query on the category archive page.
Try loading the page with or without Types and in both cases you will see (in the debug.log) that the expected posts (including film CPT) are returned by the archive query. Hence if you switch theme to twentytwentyone you’ll see the correct results. But the theme evidently does not use the Main Query results and performs its own queries. That’s where the problem lies, so, over to the theme authors...