Ok, so it seams that steam-run is using a different ldd version:
$ ldd --version
ldd (GNU libc) 2.32
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Written by Roland McGrath and Ulrich Drepper.
$ steam-run bash
bash-4.4$ ldd --version
ldd (GNU libc) 2.30
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Written by Roland McGrath and Ulrich Drepper.
And I checked… and indeed, I pinned steam-run from 20.03, I guess because I got some strange errors before… Nevermind, I removed the pin, and now it works. Stupid me, sorry for the noise.