Some address book birthday entries not showing in calendar

Hi there,

I just noticed that a few of my address book birthday entries created in my Mac’s address book do not appear in my MacBook’s calendar, however, they do appear in my iPhone’s calendar (iCloud preferences: calendar is using iCloud, address book is not).

(Calendar entries created in my iPhone’s calendar are being instantly synchronized with my MacBook’s calendar – so synchronization basically works.)

For testing purposes I exported single vCards from my address book. Those contacts that do NOT appear in my calendar are being exported as "Kontakt.vcf" (German macOS) and appear to be empty (size 0 Byte, no content when opening with BBEdit. Those contacts that DO appear are being exported as "Kontakt.vcf" "First_name Last_name.vcf" and do contain information as expected.

What is going on here?

I have been using the adress book for many years on a few different machines and mac OS versions. Is there a way to "clean install" my contacts somehow?

(macOS Big Sur 11.2.2., MacBookAir10,1)