SOLVED: Why is the transcoding bitrate limited

[bagno]

Ah, my apologies. I forgot the mention that the source/target is 4K, not 1080p.
I tried setting the preset to medium again, and now I'm left with only the second problem - see the video below (recorded before the change, but the symptom is the same):
https://www.youtube.com/watch?v=i5Xaic49ZsI