Birthday isn't showing up on my Calendar for certain contact

I am using 15plus and on ios26. Usually when I save a contact and they have their birthday on their contact card, it automatically syncs to my calendar; no need for me to input their birthdays manually. However, I just noticed this person's birthday isn't showing up on my calendar anymore. I blocked them and unblocked them after a while and now their birthday is not showing up on the calendar. Even when I check their contact details, the birthday is not there; like it's gone. I don't want to input it manually I want it synced.

was me blocking them the reason? or did they block me maybe they deleted my contact? or was it a bug?

[Re-Titled by Moderator]

Original Title: birthday syncing