is the "realtime-privileges" package installed?
If not, then install it, add your user to the group "realtime", log out and in.
If this shouldn't be the issue, is 99-realtime-privileges.conf the last config providing the memlock configuration or does another package follow and override the setting?
$ grep memlock /etc/security/limits.d/* /etc/security/limits.d/10-gcr.conf:@users - memlock 1024 /etc/security/limits.d/99-realtime-privileges.conf:@realtime - memlock unlimited
A memlock value in /etc/security/limits.conf is null and void, if any package should have "dropped" something counterproductive to /etc/security/limits.d/. An admin needs to check this after _each_ upgrade.