You seem to have mis-spelled "works". (-: The program has made the system calls that you told it to, and they have succeeded, setting the soft limit that you instructed.
Indeed, there's nothing in that strace that points to a dysfunction in s6-softlimit. (But it's an excellent illustration of why Linux users should switch from glibc to musl. :))
Jan, do you have any more evidence pointing at s6-softlimit as the culprit
in your issue? -- Laurent
