The plumbers said they were working. Water was flowing.
That doesn't mean anything.
If the system is wired correctly, the operation should be;
a. Nest or whatever thermostat you have connects power to the valve motor
b. valve opens so that water can flow
c. when valve fully open, a switch in the valve activates the boiler/pump.
d. when power is removed, a spring in the valve causes it to close and the switch in the valve opens so the boiler/pump stops.
It's entirely possible for valves to open and the switch to not work, or only work sometimes, or the switch to fail closed, or even to open and then close if the motor/spring assembly in the valve is defective. Valves can also jam open, closed or partially open. Sometimes intermittently. The possibilities are endless.
That also assumes the system was wired properly in the first place, which is by no means certain.
Ultimately any of these problems and any others can easily be identified by checking for voltage at the various terminals in the wiring centre. Randomly guessing and making changes will not solve anything.