SP18 is indeed the service pack that is running (checked productinfo, System Overview, and /libs/system).
Removing SP12 from /libs/system does not work. However, I can remove the package either through crx-de (/etc/packages/adobe/cq650/servicepack) or package manager UI (Select SP18 package, then More dropdown. Click Other Versions, open SP12, then More -> Delete.). I no longer get the message on the package "A different version seems to be installed..."
However, I am still getting these entries in error.log during start up and the SP18 package will always show at the top of package manager. It's confusing, because someone might think SP18 was recently installed. Any thoughts on that?
12.09.2024 18:07:10.387 *INFO* [OsgiInstallerImpl] org.apache.jackrabbit.vault.packaging.impl.ActivityLog adobe/cq650/servicepack:aem-service-pkg:6.5.18.0: CREATE
12.09.2024 18:07:10.387 *INFO* [OsgiInstallerImpl] org.apache.jackrabbit.vault.packaging.impl.ActivityLog adobe/cq650/servicepack:aem-service-pkg:6.5.18.0: UPLOAD
12.09.2024 18:07:10.916 *INFO* [OsgiInstallerImpl] org.apache.jackrabbit.vault.packaging.impl.ActivityLog adobe/cq650/servicepack:aem-service-pkg:6.5.18.0: EXTRACT_SUB_PACKAGES ()