If you have the circulator wired to C1/C2 in the triple aquastat, the behavior you described will happen if the boiler temperature drops below the low limit setting. This is intentional.
If you want to prevent this, you can do a few things.
1. Reduce the flow rate of the zone.
2. Increase the feed rate of the stoker.
3. Reduce the low limit setting (assuming you do not use tankless coil).
4. Wire this zone off a separate relay.