It is kinda less known that the Maddog-B arm omnipods allow for "arm actuators" unlike the other non-Maddog-B omnipods. It's unclear why that is...
Anyways, you can "inject" the arm actuator into non-Maddog-B arm omnipods (assuming you aren't invoking the rules of removal such as equipping a PPC or non-MG ballistic).
To reproduce:
1) Select any Maddog.
2) Equip the Maddog-B's arm omnipod (left or right arm is just fine - swapping the left one is easier since you won't have to rotate the mech to visually confirm it) if it isn't already equipped.
3) Enable the arm actuator (obviously you need the lower arm actuator enabled and you may need to remove any PPCs to enable it).
4) Swap the Maddog-B's arm omnipod with a different omnipod (it doesn't matter which omnipod).
5) Save the config as is.
You can see the arm still be there even though it's not a Maddog-B omnipod.
To "fix" or reverse this, you can just remove (and readd) the lower arm actuator and see it disappear (you could also equip an ERPPC) and save.
I'm not sure of the implications of the bug, but while it probably isn't factoring into the crit calculations (untested though), it may increase the hitboxes unnecessarily. The game SHOULD auto-remove the arm actuators when swapping... however in this instance it is not the case.