I JUST had this same issue. Even worse, the Sendgrid "sample renderer" supports flex, but Gmail doesn't, so I didn't notice there was a problem until I started

I JUST had this same issue. Even worse, the Sendgrid "sample renderer" supports flex, but Gmail doesn't, so I didn't notice there was a problem until I started sending myself test emails, and then had to rewrite the whole template with tables. Madness!