A curious set of events!
Until recently my Home Office setup was an iMac Late 2013, and I travelled with a MBP 2015.
I have an Android phone, so all my contacts have both iCloud & Google cards, which I linked in contacts so that any change on one platform was mirrored on the other.
Then the iMac got stuck on Catalina, due to Apple’s strange attitude towards supporting older models (Whereas Windows 10 seems happy to support our incredibly ancient PC!). I was fine with that until Office 360 also dropped support for Catalina. To try & remedy this I updated the iMac to Big Sur using OCLP which seemed OK at first but then caused a bunch of problems, including my contacts slowly de-linking, & Birthdays in Calendar spamming up to eight instances . I tried signing out of iCloud & Google on all devices, then signing back in, & that helped for a while, but then the contacts would start de-linking again. In the end, I dropped back to Catalina, did a Time Machine reinstall of all apps etc. got it running, did another back-up, sold my iMac , & got a 2023 Mac mini. A Time Machine transfer got all my iMac info onto the Mac mini & on sign-in to iCloud & Google my Birthday multiples in Calendar were gone, but I still had iCloud & Google contacts de-linking or refusing to link. I contacted Apple, as my Mac mini was still covered by the start up free cover & we tried pretty well what I’d already done, but with the same results.
In the end, I linked all I could on the Mac mini, went to my MBP, linked any I could there. Back to the Mac mini (I only had one computer on at any time) & any that were not linked, I deleted one of the cards, then dragged it to iCloud to produce an iCloud contact (Interestingly, whichever of the two instances I deleted, it always seemed to be the iCloud one I had deleted - there is no way in “All Contacts” when you have two identical but unlinked cards to see if the individual card is iCloud or Google).
Now, all cards are linked between both iCloud & Google & none have de-linked! Fingers crossed that it remains this way!
Just thought I’d share a curious train of events to anyone interested.