I make my slouchie beanies from the brim up, rather than starting at the crown. I’ve noticed that often this brim will curl up a little. Not a terrible problem but it can be annoying and un-pretty.
I recently started a new slouchie beanie for my daughter (in soft rainbow yarn!) using a foundation double crochet (single for US crocheters), and I’ve noticed that the brim isn’t curling. Yay!