Oh, at least some regularity is revealed. I need to check it myself, I’ve gotten a mistake when combining the contacts, there’s no power ...
Additive:
If the contact is without a birthday, you can add it, BUT then when viewing it gives something like
Birthday 1975-06-09T00: 00: 00.000Z
It's cool, of course, to have the DR in milliseconds, but most likely there is a banal application error that does not understand the absence of the "time" value in the record (in the corresponding .vcf file there is a line "BDAY: 19750609")
And it seems to me that you just need to update the application "contacts", because programmers are also people: o)