Did iOS 18 or Yahoo app change my calendar?

Did iOS 18 update or Yahoo app update change my default calendar?

I recently installed iOS 18 and also updated the Yahoo app. I noticed after the iOS update that new meetings I created on my calendar were getting created on a Yahoo calendar. This isn’t what I wanted so I changed Settings to restore new calendar items to the Apple calendar.

However, I do wonder what caused my Settings to change, the iOS 18 update or Yahoo app update or something else??!