Hiya @florent-bossuyt ,
Thanks for using FusionAuth.
The upgrade to 1.59.0 added support for phone number as a first class identity. While I don't know for sure, it is possible that impacted lastLoginInstant. I'm not aware of any bug filed.
It's also possible you can find the data previously in the lastLoginInstant in the user.identities[x].lastLoginInstant value, available when you retrieve a user.
Since this is not a known issue, there's no documented procedure for reconciling the data, unfortunately.
Here's one approach, though.
If you have a database backup of the 1.57.0 release, you could stand up a FusionAuth instance pointing to that, and find all the users with a lastLoginInstant that is not null (set A). You'd then run the same query against your prod instance (set B). Then find the folks in set A who are not in set B (call them set C). Set C are the people you need to update.
Unfortunately, you can't update their lastLoginInstant directly. But you can fake a login using the passwordless APIs, which should set it. That is outlined here.
So you'd want to perform that for all the users in set C.
Hope that helps.