Imported schedule automatic date change

When opening an imported file dates change on some activities.

Displaying start/finish date variation columns, even before assigning a baseline, some activities show a variation.

When creating a baseline of the opened schedule, selecting making the current schedule a baseline, even more activities show a variation in dates.

The schedule is not F9 processed or anything, just opened.

Does anyone know why this is and how to stop it?

Thank you.

By the way, this only happens with schedules imported from another system. I suspect it's something to do with settings being different between the two systems, but have not been able to identify the culprit.