Thanks. VPSes from IONOS. It's doing it on another too (AlmaLinux 8.10, Plesk Obsidian 18.0.60 Update #1), but not on a third (AlmaLinux 9.2, Plesk Obsidian 18.0.61 Update #4).
It does not appear to be causing any issues, outbound emails are being signed, so I'll leave it for now - I just...