Odd conflict with Phantasmal Force

[Dietired]

Hey, I had an interesting interaction last night where an illusionist player attempted to use Phantasmal Force against a Burrowshark riding a Bulette. In Princes of the Apocalypse the Burrowsharks have a mental link to the creature they are riding in which they share perceptions.
When the Bulette was affected with Phantasmal Force the player tried to convince the beast that its owner was stabbing it repeatedly with his spear.. But with the shared perception I couldn't justify that it would be convinced, so instead I allowed the damage, and said that they were stunned for their turn from the confusion of trying to sort out the conflicting perceptions.
How would you have ruled in your games?

[Aett_Thorn]

Totally not RAW, but if this was me, I would have let the player choose a Phantasmal Force effect that affected both the Burrowshark and the Bullette. With their shared perception, it might have affected both of them at once. But I would have let both make a spell save, and if either of them succeeded, then the spell would have failed.

[Desteplo]

I would have had both make the D.C. save. Or gave the animal advantage.
-just because of an odd situation try not to take away an effect. Give negative or positive attributes like advantage of disadvantage. Don't take it away
-for the remaining turns the mount would still think it's real. Even if they share perception it would "rationalize" why its master can't see it. As in, he can see in a different plane and it's hurting him.

[jas61292]

I think the biggest issue with this is that phantasmal force simply cannot cause the effect the player desires in the first place. There would be no conflict, because the spell simply creates the illusion of an object, creature or phenomenon in its mind. It does not alter its perception of existing objects or creatures. Sure, you could make an illusion of its rider attacking it, but it would be a duplicate of the rider who is just riding it normally. The actual rider would still be perceived, and as it is both not attacking, and has a mental link, the worst that could happen is that the beast thinks it is being attacked by an imposter or clone (if it had the intelligence to understand such things).