A siphon break in the head to holding tank discharge line?
A siphon break in the head intake (the the line between in the sea water through hull to the head) needs to be looped high with a siphon break.
Not so the head discharge to holding tank line. In fact, no sanitary line on the discharge side needs a siphon break. If there is a holding tank in the mix a siphon threat via a holding tank is impossible because the holding tank is already vented thus no siphon threat is possible.
In short, no sanitary discharge line can be exposed to the boat interior.
Your arrangement is providing unpleasant consequences. Remove this device pronto. But -- double check that your holding tank to atmosphere vent line is open - no kinks, no vent hose failures either.
Charles