I also encountered this just now, while editing my homebrew. I took a look at this table guide thread for comparing different table types depending on class combinations.
From what I conclude, any combination of"Head", "Alt1", and "Alt2"class values will default into a "Borderless" class.that ignores all other class parameters (even ones such as padding and sortability). Not sure specifically why that happens, but I'm just posting this in case anyone wants to immediately fix their tables' aesthetics. (That is, avoid combining those three class values)