⟨07nazfib⟩ How MJ executes Principia nodes is this:
- It aims at the node
- At T-{lead time}, it starts measuring dV expended
- At T-0, it throttles up the engine
- When dV expended is equal to dV required from the node, it sets throttle to 0 and ends.
Your ullage rockets give the stage approximately 22 m/s of dV; this is all expended before MJ starts measuring it. Therefore, I'd expect MJ to overshoot your node by 22 m/s. 
Try the following:
- Set the "Lead time" in MJ's node editor to something like 30 seconds, to get the ullage rocket dV included in the burn
- If you're spin-stabilizing, you need to check "Inertially fixed".