I almost hate to report this because it is such a small, unlikely thing.
If I use the Dynamics popover to enter mp (under sax solo), Dorico does recognize the parenthesized text as a suffix and adds it to the suffix property. That’s great.
However, for some reason if the last character of the suffix is a right parenthesis, it is ignored. I can’t enter it at all through the popover. But if I go to the property panel, I can enter the full string with closing parenthesis.