In fact, if your media storage is not on internal hard drives within the server itself, do disable the following check box:
Settings - Server - Library - ‘Empty trash automatically after every scan’
Indeed all my movies are on a external HDD connected on USB but it didn’t cross my mind since I would have expected all the movies to change the date added, since they are all there.
I have disabled ‘Empty trash automatically after every scan’ - hopefully this fixes the issue.