The definition of the word "assure" does indeed suggest that removal of doubt in the listener is expected, but there's an important thing to remember: a speaker cannot control the level of doubt in the listener, only influence it. So we have some contexts to consider:
If the narrator is reliable and omniscient or if the listener is the narrator, Bob assuring Sally likely truly does mean that Bob's assurance has resolved Sally's doubt
If the narrator is not reliable, not omniscient, and not the narrator, Bob assuring Sally simply means that Bob is doing his best to resolve Sally's doubt; since the narrator can't read Sally's mind, there's no way to know if the assurance had the desired effect
So do understand that context matters for this word. Sometimes meaning must be interpreted according to the speaker's intentions, while other times it can be interpreted as truly having the implied or desired outcome.