I remember this being talked about decades ago as a bug that's too hard to fix.

But as it stands:

a) PC initiated combat
b) 1st full round of combat where the NPCs assist
c) 2nd full round of combat where the NPCs do not attack
d) 3rd and subsequent rounds of combat where the NPCs do get attacks

Are you saying at b) they get the equivalent of two rounds of combat and that is balanced out by zero rounds of combat in c)?

I don't think that's the case but if so that's a bug either way. NPCs are either getting 2 rounds followed by 0, or 1 round followed by 0.