If there's no contact between the bullet and bayonet, Mobuck may have nailed it.Adding that much weight to the muzzle could easily change the barrel harmonics. If nothing else, putting the weight so far out front will change the recoil movement and move POI.
M40 106mm recoilless rifle?Are there any rifles that have no POI shift with a bayonet attached?
I think it has to do with the a difference in air pressure/gas expansion hitting the bayonet as the bullet leaves the barrel.
The barrel harmonics can change. I think the weight of the bayonet has a definite impact.
However, the weight of the bayonet, like adding a suppressor, causes barrel droop, hence why the shots will normally be lower when you add a weight to the end of the barrel.
In the military they are not installed unless it's gonna be close contact, very close. In that case I doubt the thing will effect the accuracy enough to make a difference.