How can I shareplay a Paramount Plus show with a family sharing subscription?

My partner is out of town for a while. She’s in my family group. When she checks subscriptions, she can see Paramount Plus listed as shared by me.

However, when we try to start a SharePlay session with a Paramount Plus show, she gets a notification that she needs to subscribe. And it only works at all on her phone — if we try to watch on our Apple TVs, she has no SharePlay option at all, and I can’t see her in my SharePlay options, even during a FaceTime call.