Right now, the ISO build script will always attach a -dirty prefix to the build commit hash. In development builds, it's useful because it tells if it was actually built from a specific commit, or there were local modifications.
However, for release builds, local modifications can be normal and even expected, e.g. when building custom flavors or using custom default configs. We also use a different MOTD for official LTS builds, which also marks them all dirty even though there are no functional changes.
I believe the dirtiness check should be limited to development builds only.