Based on an alert I received from BusyCal today, I suspect that what you are seeing is due to a change on the Google side. Google has done something with birthdays/anniversaries that the 3rd party services can’t turn off.
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This issue has been so annoying, I use an iCloud account for my Primary calendar but for a FEW things and misc invites I use a Gmail account so I need it showing in Fantastical and the duplicate birthday between Contacts App + Google Calendar showing birthdays. I have about 85 contacts in Google with birthdays added, I may just remove them from their contacts so I can deal with this as I haven’t found another fix.