A French nasal vowel is one gesture, not two sounds. To say bon you do not say beau and then an 'n' — you lower the soft palate (the velum) so the airstream vents through the nose while the vowel is sounding.
When the velum drops, the nasal cavity couples onto the mouth. Acoustically that adds a low nasal pole near 270 Hz and, above it, an antiformant — a notch that eats energy — while the first formant broadens. The vowel goes hollow and hummed.
The tongue barely moves between beau /o/ and bon /ɔ̃/: it is the velum that carries the meaning. Slide it and watch the spectrum pull away from the dashed oral ghost.
This is why nasal vowels are the classic stumbling block: the change is invisible and gestural. Make the valve a thing you can grab, and the cause becomes something you can see.
Something in the simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.