I've noticed on the initial rides on my Street Bob that, compared to my previous Dyna Low Rider, the rear brake is next to useless... is it just mine, or is this a common "feature"?
My 2018 Fat Boy is the same . .. I doubt it would pull up a Lambretta ridden by a miniature poodle wearing a light collar!
Faulty brakes! Air trapped in the system could be your problem, request to have the system flushed for your safety.
I heard today there is a recall on Street Bob's for a brake problem - I think it was.
I saw earlier this month a recall (and stop-sale) for Street models (as opposed to the Softail Street Bob / FXBB), but it's pretty unimpressive performance!