Yes. I do not know your translation, but "Moja matka przynosi nam kubek herbaty" is correct. And even more correct than translation above. You should report it.

It's not wrong per se maybe, but sounds rather strange. I see no reason to put "nam" before the verb, that's not natural.

I don't get something either, because the only sentence that I could find was "My mother brings us a mug of tea." and this discussion is under "a cup of tea" (much more idiomatic, but not exactly the direct translation).

I will change "My mother" to "My mom", which is a better rendering of "Moja mama". But I don't know if this isn't some kind of a ghost sentence... I though that so-called ghost sentences were a result of our lack of knowledge about Incubator, but "My mother brings us a cup of tea." really doesn't seem to exist anywhere anymore.