There does seem to be a bug - if you drag the tongue to the bottom left-most position, close the lips and bring them back as far as possible, raise the larynx all the way, and lower the soft palate all the way, the synthesizer starts making some kind of inhuman screeching sound (which is definitely not physically accurate).

My best guess for why is that the simulation parameters are set up slightly wrong in a way that is mostly fine for normal simulations (not an audible amount of distortion) but quickly goes off the rails on weird cases like the aforementioned.

Overall a really nice app though; honestly I was kinda blown away by it.