Just had a very similar issue: 2019 Max Limited. Went on vacation for 10 days, battery wasn't fully discharged, but after it sat for the 10 days the AC blew external temp air. Checked blend door operation by switching to heat, and it blows HOT, so that's not the issue. Checked the refrigerant at the low-side port and it read low, so I put part of a 12-oz can of R1234YF in, and it's up in the green for pressure, but still only blowing 'cool(ish)', vice ice-cold like it did when we parked it.
Next steps for me, and what you might look into:
-I have a set of A/C line gauges on order, so I'll be able to see the static pressure on both sides when the compressor isn't on, then both the high and low pressures when the compressor is on. Gauge on the fill adapter shows ~110psi on the low side w/o the compressor on, and ~40psi (mostly guessing since the gauge isn't marked very well) on the low side with the compressor running. I went ahead and ordered the gauges so I can check both sides simultaneously when the compressor kicks on to see if it's over-filled.
-I need to check on the active vent shutter operation to see if they're staying stuck closed when the truck is on. With zero air-flow, it would pretty much kill any ability of the radiator or condenser to dissipate heat.
Interested to see what other steps you take to see if any of them fix the problem.
