120V on a 240V, 30 amp circuit? Brain has zero volts
Ok this is probably a simple issue. You have a double pole circuit but the thermostat is only single pole and you read 120v  not 240 as you expected.
With baseboard heaters it is common to only break one leg.
With one leg open no current can flow so no heat.
The heater is required to be grounded so measuring the voltage with the heater turned on providing heat at either terminal to ground will show ~120vac.
With the heater heating the voltage from L1 to L2 (the heater connections) will show 240v.
Now here is what trips up non electricians. With the thermostat off each leg still reads 120v to ground the voltage coming through the heater has no voltage drop because there is no current with a normal high impedance meter.
So a 240v heater can be controlled with a 120v line voltage thermostat.
To know if your heater is 120v or 240 measure the voltage L1 to L2 at the heater when the thermostat is turned on (the 2 wires connecting the power to the element) if you read 120 it’s 120 if you read 240 it’s 240.
You have found out you have a 30 amp circuit, this is quite common and #10 wire is normally used. A single pole thermostat can control 120 or 240v if when the thermostat is off you have voltage (120v to ground) at the heater this is another indication it is 240v.