A similar gap exists for a wide range of high-mass stars, in the same formation. If you look in most sectors, a g-mass star (e.g. Blah AA-A g0) will be something really heavy, like an O-class, a B supergiant, a black hole, or a Herbig protostar. If you look in a sector along the cross, AA-A g0 will be something like a 1 solar mass T-Tauri, or a light M-class dwarf.
The best theory I've seen for it is that the stellar forge is programmed to suppress high-mass star generation near the bubble (because the non-procedural HIP, HD, etc. stars fill that role and otherwise you'd have far too many) but due to a bug extends that suppression along the major axes from the bubble as well.
It's a bug which has become more obvious since 2.2 with the ability to collate scan data from thousands of commanders, though it's been known for longer.