How cool should an air conditioner keep an upstairs apartment?

Refrigerant is the correct term for "coolant". Call the techs again and have them check the refrigerant charge pressures.

Upstairs should be set 2 degrees higher than downstairs to compensate for heat rise. (If you have 2 thermostats).if upstairs is unable to get temperature down within 2 degrees of downstairs, or, if your system can not provide 68 to 72 degrees downstairs, it’s not working correctly. Go outside and put your hand on the large copper line, NOT THE SMALL ONE, the line probably has black insulation on it. Push that away to feel temperature of your suction line. It should almost be too cold to hold. If just cool, call maintenances.

The closer you get to a 20 degree temp difference between the return and the supply, the better off one is. AC's work best and are often only designed to maintain about a ten degree difference between the ambient air and the conditioned air. Which is what you have. To be more specific would require knowing how old the system is, what type of systems it is, how the system has been installed, location/zone the system is in(makes a difference for sizing) humidity levels. The list can go on and on. The freezing up stopping after the filter change would point to air flow as the cause of the freezing before refrigerant being low. t could also indicate that enough has leaked out now that there isn't enough to freeze anymore. But it is still cooling so i would look elsewhere for an explanation.

For apartment AC