Looking at this logically...
Maple fretboard: adds percussive attack and brightness to a guitar or bass. Rosewood fretboard: adds a touch more warmth and maybe a little less sustain due to being a naturally oily wood, so the overtones are slightly dampened. Flatwound strings are not particularly bright, so "on paper" at least, flats + maple = possibly a good combination. Like putting a good hifi together, you need to focus on components that balance with each other. It's no different for instruments, in my opinion. On the other hand, if you want to avoid brightness and attack, and instead go for a richer, more mellow bass sound, rosewood and flats would be the better option.
Maple fretboard: adds percussive attack and brightness to a guitar or bass. Rosewood fretboard: adds a touch more warmth and maybe a little less sustain due to being a naturally oily wood, so the overtones are slightly dampened. Flatwound strings are not particularly bright, so "on paper" at least, flats + maple = possibly a good combination. Like putting a good hifi together, you need to focus on components that balance with each other. It's no different for instruments, in my opinion. On the other hand, if you want to avoid brightness and attack, and instead go for a richer, more mellow bass sound, rosewood and flats would be the better option.

