Lightroom: Catalog optimization changes metadata from "up to date" to "has been changed"

I've noticed that every time the Lr catalog is optimized several metadata randomly changes from "up to date" to "has been changed". I'm absolutely sure the photos haven't been changed in any way so I guess it's a bug with the optimization routine in Lr. It's quite annoying since there is no practical way to specifically search for the ones that are marked as changed (down arrow). Has anyone experienced this problem?