It is not normal English for a "b" at the end of a word to get assimilated into the "s" at the start of the word. (Note also that when you say "rubs", the "b"

It is not normal English for a "b" at the end of a word to get assimilated into the "s" at the start of the word. (Note also that when you say "rubs", the "b" is not getting assimilated - it's the "s" that is getting assimilated into a voiced "z")