Films won't add to a collection

[tompw]

What happens:
* I select the films in "movie" view
* I go to "add to collection", and choose the collection I want to add them to
* Jellyfin implies it's all good
* The films don't appear in the collection
* The films *do* appear in the XML file.

... I've tried removing from the XML file and re-adding; I've tried deleting the collection entirely and re-creating, but all to no avail.

The films that won't add are:
* Star Trek IV The Voyage Home (1986)
* Star Trek V The Final Frontier (1989)
* Star Trek VI The Undiscovered Country (1991)
* Star Trek First Contact (1996)
* Star Trek Nemesis (2002)
* Star Trek (2009)

(... can you guess what the collection is?)

All the films have the sort title of "Star Trek [year])", which should make no difference - but it's the only thing that makes this set of films different to any others I've added to collections.

Any suggestions?