Actually it's l am lating, he is not. מאחר is a verb and not an adjective or a pronoun. אני אחרתי, I was being late, l was "lating", its a hebrew peculiarity that for the state of lateness a verb is used. While you can translate this with a pronoun or adjective, thats not what it actually means to an israeli. מאוחר is "late", האותבוס מאוחר, the bus is late. Bus here is an object you are referring to. When speaking about yourself or others a verb לאחר is being used. לאחר is an indefinite form and מאחר is present tense masculine form, מאחרת is present tense feminine form.
Except that "I am lating" does not exist in English. I understand what you are trying to say, and you are correct, but this is one example of how languages work. There are many instances where there is no equivalent, but alternative solutions must be used.
I am also writing this, so that non-native speakers of English, who are doing this course don't think that "I am lating" is a legitimate sentence, and start reporting it, that it should be accepted.