Birthdays after pst import

[Narev]

We tried to import a large pst file of over 4gb which went flawlessly except for 40 mails that exceded the limit of 10 mb. However, after a more thorough investigation i noticed that the birthdays are way off. I can't really find a good reason why but to give you one example, befor the import outlook tells me my bday is on 15-09-1986 and after the import zimbra tells me its on 06-02-2021, which does not make any sense at all. I then continued to check the agenda, but it appears that, that is working correctly. Did anyone else encounter this problem? Or did i miss a bugreport about this, or did i do something wrong during the import? :]