When both FD and AP are off, turning FD on in air should not engage any mode, and F/D bar keeps hide until selecting a mode via MCP or TO/GA.
Right now in v1007, when F/D engaged in air, it brings HDG SEL and V/S with it, kinda like what 757/767 would do.
It's hard to pin down in FCOM, since this behaivor is not written, unlike 757/767's FCOM have clearly says V/S and HDG HOLD would engage in such condition. But lack this description in 733's FCOM is a indication it's not how it works