sorry, my fault.
I "had to" upgrade ffmpeg at some time in the past;
i ffmpeg Depends libavformat58 (= 7:4.3-2)
i A libavformat58 Depends libgnutls30 (>= 3.6.14)
i libgnutls30 Depends libc6 (>= 2.25)
and so I had a newer libc than stable.
Sorry about the confusion; perhaps a "Conflicts" might be a good idea,
though.
Thanks for your effort!