The minds of intelligent animals (and all other intelligent creatures - ones with Intelligence, rather than Cunning) are affected by Mentem rather than Animal (ArM5 p.117). The Bjornaer chapter in HoH:MC (p.22) says, however, that the minds of Bjornaer in their Heartbeast forms are affected by Animal rather than by Mentem, despite the fact that the Bjornaer retain their human intelligence. The rationale given is that the Heartbeast ability is the most "essential" of the shapeshifting abilities. To me, it seems that the author just got confused. If an intelligent Beast of Virtue (and you can't get more "essential Animal" than that) is affected by Mentem, so should a Bjornaer with Intelligence. Did this get errata'ed or discussed somewhere?
The author didn't get confused. He was reiterating what was written on ArM5 p.117, 2 paragraphs below the one discussing Intelligence and Cunning for what kind of spell affects a creature's mind.
"A Bjornaer magus in animal form is truly
an animal, so his mind is affected by Animal
spells rather than Mentem spells, and his body
by Animal spells rather than Corpus spells."
Most magic cannot tell that Heartbeasts have Intelligence. Sight of the True Form would be able to see that a Skinchanger in animal form was not an animal, or that someone under a MuCo(An) was a human. But the spell would not be able to identify a Heartbeast as a human. If Mentem magic cannot identify the Intelligence, it cannot affect it.
As for why the game designers went this route? I guess as an extension of the Hidden Shape ability Bjornaer had in 4th edition. Most magic could only affect a Bjornaer based on their current form.
I'd personally avoid the "identify" argument, as there are lots of magics that can affect something that they can't identify. Vim magic affecting demons, for example - or any magic that affects something inside a structure, regardless of whether or not the thing inside the structure can be directly identified - ie, the invisibility targeting rules.
Rather, I'd just say that the Heartbeast is so supercool awesome, you functionally ARE that animal - even if you're intelligent, you aren't a magically intelligent animal (or something else designed with handwavium), and as such you still need to be affected by Animal.
I'd say it is a special advantage of being magi Bjornaer, that when in the shape of the Heartbeast you are indistinguishable from a mundane beast (barring any other modifications, active magic, speaking etc).