**A drop from 55°F after the evaporator to the low 40s at a vent would be very unusual in an ordinary residential A/C system.** If those are accurate measurements of the same airstream during steady operation, something would have to remove additional heat along the way.

Supply air generally warms as it passes through ducts surrounded by warmer rooms or attic air. Heat passes through the duct walls; insulation reduces that transfer. [DOE duct guidance](https://bsesc.energy.gov/energy-basics/hvac-sealed-and-insulated-metal-ducts).

Several things could explain the reported readings:

- **Surface temperature versus air temperature.** If the inspector used an infrared thermometer, it measured the grille or a surface inside the duct. Shiny metal can also produce misleading readings because it reflects infrared radiation and has low emissivity. [Fluke’s measurement guidance](https://www.fluke.com/en/learn/blog/temperature/how-to-get-great-results-with-an-infrared-thermometer).
- **Different parts of the airflow.** Temperature immediately after a coil can vary across the duct. A probe reading 55°F in one spot may miss a colder stream feeding a particular branch.
- **Different operating conditions or probe placement.** Measurements taken at different times, before the probes settle, or with a probe touching metal can give an unreliable comparison. “Above the coil” also needs to mean downstream in the actual airflow direction.

A **dry grille cooled only by the passing air cannot remain colder than that air at steady state**. Additional cooling could come from evaporation on a wet grille or contact with something colder. Likewise, ducts surrounded by air colder than the supply could cool it further. Those would require specific evidence in your installation.

The useful next step is to have the temperatures repeated with air probes, during the same continuous cooling run, downstream of the coil and inside the vent’s airstream. Take several readings across the supply duct.

**These two numbers alone establish neither normal operation nor a system fault.** Evaluating performance also requires return-air temperature, humidity, and airflow.

This is a practical HVAC troubleshooting question; Home Improvement SE is an appropriate place for it.