Just tuning in a bit late to this thread... A few comments:
I'll bet he wired the ground to the neutral, or did not terminate the ground.
Ground and neutral should be bonded at one point of the electrical system (typically at the service entrance panel) and so they have 0v of potential between them. Even accidentally inverting the two the UMC would have no idea. It is not actually even connected to the neutral pin in the slightest. It is just testing the ground pin, and even if neutral were hooked to the ground pin it would have no way to know that.
I doubt your outlet was installed to code with what looks like exposed romex. That would make me question your "licenced" electrician.
That looks like romex inside of an unfinished wall. This is commonly allowed I think, though there are rules to keep people from hanging crap from it, etc... (so vertically it is often OK, but if horizontal it needs to be above a certain height or something like that)...
When you say you checked the voltage - I assume you have a multimeter? You mention that from the two side slots (the hot slots) to the bottom slot (the neutral) is 120v. That’s good. Did you try to measure from a side slot to ground (the fourth point which is more of a circle than a slot)? If properly grounded, you should get 120v. I would guess that you will actually get 0, indicating the ground is not connected. If you do get this result, just call the electrician and tell him how you tested the outlet. That would definitively prove that there is something wrong with his work.
You should be able to test as follows with a multimeter:
Hot 1 to Hot 2 (side slots) ~240v
Hot 1 to neutral ~120v
Hot 2 to neutral ~120v
Hot 1 to ground ~120v
Hot 2 to ground ~120v
Now the one thing with the install that I think might be a technical violation is that the metal box needs to be grounded. The ground wire should be connected to the metal box with a grounding screw in addition to the receptacle.
I can understand why the electrician might be a bit grumpy to come back out as this is an extremely simple install and he might have already checked it with a multimeter. There is a good chance the issue is with the UMC itself. As others have said, find another 14-50 receptacle that is known good and test yours with your car. Also, another option would be to find another UMC and test it with your receptacle and car (or someone else's car). I know my local Tesla group would be more than happy to find someone to come help you if you were local (it is a good community!). There may be something similar near you.
But now on to the more concerning and scary possibilities:
It is possible that something is fundamentally wrong with your electrical system in that perhaps your neutral and ground are not properly bonded together. The UMC would pick up on this and refuse to charge. Though I would think that would also impact 120v charging. Other possible concerns would be a "loose neutral" between your house and the utility transformer. It is possible the UMC could detect this by noticing diverging voltages from the two hots to neutral, but it would not be able to detect that on the 120v charging. I am not sure what all it checks for. Regardless, that is an extremely dangerous situation (for the whole house, regardless of EV charging) and would need to be resolved ASAP.
But there is a good chance the issue is just a bad UMC (or something wrong with your car).
Oh, and if you do feel comfortable enough to safely take the panel cover off then post a lot of pictures here of the wire as it comes into the box and its terminations and of the box itself and we may be able to provide advice. One thing we will be looking for is the "neutral to ground" bonding screw or strap. Modern ones are green in color (screws).
Good luck! Please report back on the resolution!