Here’s what helped me to get the final cutscene, finish the quest and get a new one.
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After you’ve done brewing a cure don’t go to Merhojed, instead go and talk to Radzig at a camp near Talmberg (fast-travelling is OK) - it’s marked with a campfire on a map and is directly to the west of the town. Tell Radzig about the plague. You can get the quest about silver mines while you’re at it - and also just to get a free savegame.
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Now go to Merhojed, talk to Nicodemus, give the cure to everybody in need, finally ask Melihar to open the barn, give the cure to the bandit, wait a few hours, get all the info you want from the bandit (it doesn’t matter what you’ll decide to do with him).
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Now, without waiting for the “Pestilence” quest to complete, immediately go back to Radzig’s camp and talk to him about the events at Merhojed. Cutscene works.
It appears that the bug at the time of writing is related to you not telling Radzig about the plague before you get all the information from the bandit - hence why when it’s bugged and the cutscene does not work you can only tell Radzig about the plague even though you’ve resolved the quest.