This is the one blemish (in my view) on fastmail's record. I love them otherwise.
I initially came to the same conclusion as you, but after receiving one too many badly quoted HTML emails (and after some lost hours unsuccessfully trying to understand how fastmail web, gmail, thunderbird and ios mail.app do HTML quoting (hint: they're all subtly different)), I decided to double-down on format=flowed.
I initially came to the same conclusion as you, but after receiving one too many badly quoted HTML emails (and after some lost hours unsuccessfully trying to understand how fastmail web, gmail, thunderbird and ios mail.app do HTML quoting (hint: they're all subtly different)), I decided to double-down on format=flowed.