In this particular case there's no 3D work involved, it's indeed a "bug" or sort of.
The G4M1-11, according to Wikipedia, got introduced in April '41.
In Summer '42 the propeller hubs were introduced (depicted by our G4M1-11 ingame).
In Spring '43 the outer half of the tail cone was cut to improve tail gunner's field of fire.
In August '43 the V-shaped cone was introduced (like the one of the G4M2 model).
The Stock G4M1-11 has the "old" tip-shaped cone which needs to be rotated by the gunner.
The Ultrapack G4M1-11 originally had the "cut" cone, but due to 4.13 code having been ported back, we have an overlapping mesh here. Easy fix.
However, we might instead fix it all at once.
That means, give all three cone options to the G4M1 (we could replicate loadouts, with "early_" designation for the tip shaped cone and "late_" for the v shaped one).
And while we're at it, re-introduce the bomb fairings for loadouts that have them & re-arrange the loadouts with smaller bombs on the G4M1, and finally: Give the G4M2 the sliding bay doors which it deserves.
This will be a bit of work.
Mike