So my question is - is this really supposed to be leveled out and if yes, is it possible this didn't happen because disk which failed and was replaced happens to be spare disk and leveling works on non-spare ones only? Anyone with more experience and insights on this?

Hrvoje,

I checked with the Hardware team on this and they stated that is exactly what the issue is.  Once the disk is brought into the disk group and reconstructed the firmware will update on it.