The issue seems to appear only if the event meets the following criterias: 1. Lays in SUMMER AREA of Daylight Saving Time (from approx mid-April to mid-October); 2. For recurring events (like birthdays, anniversaries) the start date lays BEFORE 1980.

That looks pretty ridiculous, but this two criterias drive the process.

So, in Outlook I selected all birthdays on contact page and changed everything to 1980 or later. Of course, no more tracability of age.