This seems like a recent regression most likely when the pinning of collections was added.
Using android 8.17 on TV or mobile goto a movie which is part of a large collection, say Dr No from the James Bond collection.
Scroll down within the movie till you get to the Collection row.
Then on that collection scroll to the right. You will see only 10 movies from the collection are shown. There should be 25 within my collection.
Can supply a screenshot if needed but it’s pretty objious and easy to reproduce.
EDIT: Just check plex on the roku and is works correctly on that app. Scrolling shows all the movies within the collection. Looks to be an android specific bug/limitation!