The 4.19.18 release of OCP includes a machine-os-images container that contains an SCOS ISO instead of RHCOS. 4.19.19 is not affected, and nor are earlier versions of OCP.

When creating an agent ISO with the agent-based installer, this results in the warning:

WARNING base ISO version mismatch in release payload. Expected version 9.6.20251015-1 but found 9.0.20250510-0

and the generated ISO fails to boot. It is to be expected that either adding new BareMetalHosts to the cluster or adding new nodes using oc adm node-image create would also fail for clusters on 4.19.18 (regardless of the version they were born in). Upgrading the cluster to 4.19.19 will resolve the issue.