As stated by Timatic, the database used by airlines:
Visa required, except for Passengers with a C visa issued by
Bulgaria, Croatia or Cyprus valid for the period of intended
stay. They are visa exempt for a maximum stay of 90 days.
So it only has to be valid on the day you enter Romania; there is no requirement that you enter Bulgaria first.
It is, however, likely that you will face check-in/gate staff not aware of this rule, who will try to refuse you boarding on the flight. If this happens, tell them to check Timatic, which says that a valid Bulgarian visa is valid for Romania.