What is your model Apple TV? Identify your Apple TV model - Apple Support
Did you subscribe to Paramount+ in the Apple TV app, in the Paramount+ app, or with Paramount+ directly, or somewhere else?
Apple TV app Channels have a different type of subscription than content provider apps, even if both can be subscriptions through Apple. Those types of subscriptions are not exchangeable as such, they have different terms and conditions, different perks and limitations.
What happens if you access those shows not through Up Next, but directly from the Paramount+ Channel pages, or from the Paramount+ app? Once done, that should update Up Next as well, so it may work again for the future.
Or navigate to the Details page for a show, scroll down to How to Watch, then select the wanted content provider.