Streaming

The Apple TV app keeps track which episodes you have watched, and the content provider that was used for that. It may get confused if you watch episodes out of order, or if you watched some episodes through a secondary content provider, or if the episode numbering differs between content providers, or if you continue when an episode became available from a secondary content provider but not yet from the intended content provider.

If you notice a mismatch, then you may set the intended content provider on the Details page of the show or episode, when you scroll down to How to Watch.

Else try removing the show from Up Next (click-and-hold the tile, then select Remove). Then continue watching from the library, so that it will be added to Up Next again automatically with the intended content provider linking.

As a last resort all of your watch history (account property) can be reset, which would also reset Up Next.

Settings﹥Apps﹥TV app﹥Clear Play History﹥

There currently isn’t a setting to force a chosen content provider. It seems intended to work fluidly for all available content providers. You may give feedback about that.