We do use systemd to manage the bird processes. Do you have it successfully creating core dumps on crashes with systemd? - would you mind sharing your unit file?
This does not work, because ulimit does not set system-wide limits, but
limits of the current shell (and it subprocesses). That is why it worked
for sleep command.
But if you run BIRD from systemd, then systemctl command to start/restart
BIRD just say systemd to start it as a child of init (systemd), not as a
child of the current shell. So it will not inherit ulimit of the current
shell. You likely need something like modifying BIRD unit to change
ulimit.