Your RPZ needs servicing
That sort of continuous spitting or streaming from a Reduced Pressure Principle/Zone (RPP or RPZ) assembly with nothing going on downstream is a sign that the assembly needs to be serviced. In particular, Watts' own troubleshooting guidance video on the topic says that if the relief port is spitting, dripping, or streaming continuously in a no-downstream-flow condition, one should close the downstream valve on the assembly, then open the most-downstream (#4) test cock with a standard screwdriver to relieve pressure on the chamber. If the flow from the relief valve drain stops, then that's a sign the first check is fouled or sticking; if it continues, then the relief valve itself is not seating.
Either way, you'll need to either get a rebuild kit and service the RPZ yourself with it following the instructions in the video or installation manual, or have a plumber or irrigation technician qualified as a backflow preventer tester fix it for you. Some local codes may require that a qualified tester perform the repair, even, and will also require in that case that the unit be tested regularly by said qualified tester.
P.S. if you haven't a clue about finding someone who's qualified to work on backflow assemblies and don't feel comfortable DIYing this, call your water utility! They'll be able to get you a list of referrals at minimum, and may even be willing to send a technician out to test it themselves, depending on utility policies.