Yes, the behavior is the same across applications between the server web app, my phone app, and LG TV app.
Yes, Cars 1 is a member of multiple collections.
Cars 1, 2, and 3 are both in a “Cars” collection set to ‘Hide items’ and “Pixar” collection set to ‘Hide items’. Cars 1 is also in a “Tom Hanks” collection that is set to ‘Hide Collection’.
When I changed the Pixar collection settings to turn off ‘Hide items’, then Cars 2 and 3 reappeared. So perhaps the Multiple Collections are conflicting.
However, there are other items in a single Collection that are also not responding to Collections ‘Hide Items’.
Not much you can do…
There’s a relatively long standing feature suggestion to change the “hiding items” behavior for collections (e.g. let “hide” take precedence over showing items). You can vote for it and hope Plex will at some point implement it.