I have now actually read Flemming's 1929 manuscript that first described penicillin [0]. It is a careful and well documented scientific report describing the action of penicillin on various species of bacteria, how to produce it, and some of its chemical properties. It describes how penicillin can kill bacteria isolated from the throats of nurses, and shows that it has low toxicity in mice, and is possibly safe for use in humans: "Constant irrigation of large infected surfaces in man was not accompanied by any toxic symptoms, while irrigation of the human conjunctiva every hour for a day had no irritant effect."

It is far from having a "lack of reproducibility" and in fact allowed others to quickly and accurately replicate his discovery.

https://pmc.ncbi.nlm.nih.gov/articles/PMC2048009/