Heartbeats won't catch UI freezes, which I've seen on phones. Hopefully, those would result in a drop of the input signal rather than it locked in its prior state, though.
Heartbeat would hopefully also include a counter, so that a locked input signal would result in the vehicle rejecting the heartbeat (sees that each message is the same instead of incrementing the counter).
No guarantee that's how they implemented it, though.
No guarantee that's how they implemented it, though.