If a note already has a breath mark, adding a fermata to it works however the fermata sign on this note will not be displayed. (if the piece is for multiple instruments, fermata appears on other staves).
It would be great if you can make it so that both fermata and breath marks can be attached AND displayed to a single note. The reason is that a composer might want to indicate a short pause/breath after a fermata. I know that there are work arounds to achieve this but it would be nice if Dorico could by default do this.