Thank you Chris and Cathal for your answers. I will look into the 'hard coded' idea a little bit more but I don't think this applies here as both leads are basically the same: They were both created the same way, all fields are set the same way etc.
I attached two screenshots to further illustrate the issue. As you can see, both leads were created as part of our initial SF sync.
Then, 3 months later, both were included in a email campaign. A spreadsheet upload with updated data changed the middle name initial for both contacts. The change for one lead was synced with SF following the campain, the other lead was never synced.
Please note: we have no "sync lead to SF" step in our workflow for the campaign. The only workflop step of the campaign is "send email." So I assume that one person was grabbed by the global sync, the other one was not.
Any help is much appreciated!