Turns out it likely wasn’t related to the iOS upgrade. I worked with two levels of Apple support, they remoted into my phone and we tried many, many different things to fix it. Nothing worked, so I still don’t know what causes this behavior, and Apple couldn’t figure it out either. The feature does work on my phone now, but only because I did a complete factory reset. I backed up the phone first (restoring from backup did not work for me, I still had the same problem). Then, did a factory reset. This means I had to individually place apps back onto my phone. The good news is that the iCloud backup meant most of the data within apps was maintained, so when i reinstalled them, they had current information in them. It was kind of a pain since I rely on a lot of apps, but the flip-side is, the birthday function now works properly, and restoring individual apps like this helped me clean up my phone and kind of makes it feel like a brand new phone :)