All right, I got curious enough try again, and found that this is apparently caused by the static build of ffmpeg that I’m using: I used
yle-dl --verbose again, then interrupted the download immediately, ran the ffmpeg command (as reported by yle-dl) directly, and this resulted in a segmentation fault.
I then tried the same command in a VM with ffmpeg from Ubuntu repositories, and there the download finished successfully.
So the only remaining issue with yle-dl in this is the terminal being broken: that did not occur with directly-run ffmpeg despite the segfault.
I’m closing this issue though, so that it remains as documentation for the problem (and cause) as originally reported.