That doesn't sound quite right but I'm certainly no authority on heater operation. Does it cycle continuously or just once? Is the TT already at the temperature setting on the thermostat? Is a heat duct aimed directly at the thermostat causing the thermostat to achieve the setting rapidly? Just thinking out loud here. Someone here will be able to help...
And welcome to the JOF!
'18 Jay Flight 33RBTS
'02 F250 Crew Cab 7.3 Powerstroke