Diagnosis
The Load circuit is stuck closed. The most common cause is a wiring fault where the Line and Load wires are shorted together behind the thermostat or at the heater.
How to Fix It
Turn off the circuit breaker immediately to prevent overheating. Remove the Mysa faceplate. Ensure the red (Load) and black (Line) wires are not touching each other or the backplate. Ensure no stray wire strands are causing a bridge between the L and 1 terminals.
If That Didn't Work
The short may be at the heater connection point. You will need to inspect the wiring at the baseboard heater itself for a crossed connection.
Heads Up
A heater running constantly is a fire risk. Do not leave the circuit breaker on while diagnosing this issue.