Assume you have rechecked coolant level.
When the system is apart and coolant put in, it is difficult to get all the air out. An air bubble can restrict coolant flow and of course if that air bubble "burps" you will show look coolant level.
I know on our old Caterpillar 3116, I had to bleed the air bubble from the thermostat housing by cracking open the coolant line to the air pump.
The next, relatively inexpensive item is to change the thermostat and gasket. It could be bad, even if relatively new.
Ex: 2003 Alpine 38FDDS. Ex: 1997 Safari Sahara. Ex: 1993 Foretravel U240