I encounter the following bug which had disappeared time ago (about 4.7 or so) and is now back as regression I suppose. 
I have here about 15 or more contacts (some of them are restaurant addresses, some incomplete ones and one is a complete one. The complete one had a birthday date (the 27.01 of a certain year in this case) included. The other had this fiel all empty. Now all these contacts have the birthday date of 27.01 of that year attributed, although the value was empty. This is severe because it alters the data integrity of the whole database. Who had birthday when???? If you are not sure, now you will have to erase 100 birthdays (or in my case up to now 15 that are not related alfabetically or in any other way, not even over groups or associations) because the birthday data field has become "fantasy". 

Reproducible: Always

Steps to Reproduce:
1.set up a normal database with some complete contacts and one has a birthday
2. the birthday suddenly appears attributed to much more than one contact (tipicaly to about 15 to 20 contacts having the same date and year attributed. 
3.

Actual Results:  
An existing birthday is attributed randomly to other contacts overwriting the original value (null) with the birday of that contact. Birthday function is now void because you cannot know anymore which data field is true and which one has been invented by contact. 

Expected Results:  
a field that is empty should stay empty. A conctact should have never attributed false data from another contact. Values of any database should not mess up. 

There was no error message what so ever, no crash. No other rare event. 
If this happens to someone who is doing marketing with a client database with kontakt (hopefully not) this is a potentially terminal error that will lead to discontinue the use of this software.