I've noticed that when I use an import plugin (e.g. IMDB) to update a record for an actor, the "Last modified" field isn't being updated.
More specifically, I have the IMDB plugin set up to overwrite the filmography (in Preferences, "Filmography" is checked, all other fields are "boxed" so as to only overwrite empty fields), and "merge filmographies" is also checked. When I run the import, if the only change that happens is new movies being added to the actor's filmography, the "Last modified" field remains unchanged.
If the IMDB plugin is set to overwrite other fields (e.g. alternative names, birthday, etc.), and that information is available on IMDB, then "Last modified" gets a new timestamp as expected when the other field(s) are changed. But when the filmography is the only field being updated, "Last Modified" never changes.