It is obviously not correct if the hair base is baked on.
So, essentially, I cannot change it or get rid of it, I have to choose a different avatar or buy a skin to cover it.
Thanks to UncommonTruth for giving the relevant answer, albeit a ridiculous restriction.
(and, I have bought hair that came with a hair base but, unfortunately, it was not cut the same as my hair base, which still shows, and the manufacturer obviously doesn't understand UncommonTruth's explanation as they keep persisting with this same idea that I can take it off).