I had the exact same problem. I solved it by reloading an earlier save right after i told the nurse in Sasau about the pestilence, losing 4 hours of playtime. I continued the quest and did everything different. The bandit died, i took the letter and gave it to Radzig - the cutscene triggered.
I saved and tried another save, this time only losing 1 hour, right after i bought the bandit to the jail in Rattay. I reported to radzig directly and the cutscene triggered again.
I noticed one thing: it was raining when it worked. Maybe you guys try that i donât know.
Anyway, it needs to be adressed soon.
(Also the Nightingale Quest.)