Then I searched manually and found the list and it shows it is shared by showing it has 1 Collaborator.
So 2 questions/issues:
Why is the List filter not showing my collaborator lists?
What could cause my friend not being able to add any more movies if they are a collaborator & I had not made any changes at all to the list or to my Trakt account?
It only shows in collaborations if you’re invited to collab on someone else’s list. Since your the list owner it doesn’t show there. It shows there for the other person though!
For the other issue. No idea
Oh if the list has over 100 entries and your friend isn’t vip, that’s maybe the issue, for example this one
Apologies for the delay - I’ve been out of office for a couple of weeks . It seems like the management of collaborative lists (for those who are added as collaborators) in iOS is bugged and I’ll take a look at this asap next week.
As for the option to filter by shared lists - I’d suggest making a post in Trakt > Feature Requests as that seems relevant for those with lots of lists.
I’ve fixed the iOS issue with collaborative lists and the new version will be available some time this week. It’s currently available in the latest beta version.