@Elías *******
You never know what they understand, but this post was not about airport but extension at immigration office.
There they have more time and means to check, but no idea what will happen.
When the letter mentions it is valid during the employment contract or valid during the 5 year visa period, I think it should not be a problem.
But since I don't expect to stay over 180 days a year in Thailand it is a non issue for me.