Brain drains usually happen due to low or uncompetitive pay, not because of corporate ideology. Otherwise Facebook/Meta would have had a brain drain years ago.
The only other anecdote I have is an acquaintance who was early (i.e. the first web-dev) at YouTube. He _hated_ working for Google and left as soon as his shares vested. That was my first experience with someone simply rejecting a fat paycheck because _Google_ was to buttoned down for them.
People are strange (and software engineers are a whole different level of people).
It's easier to reject a fat paycheck after your shares vest, I imagine.
Still, no real data to back that claim up though.