I use an app (paid version) called calengoo. I have events on my phone going back fifteen years. It syncs with Google Calendar on all my computers and devices. It is very customizable and works flawlessly.
I had that happen on two occasions for one number in particular. The first time it happened, I removed the '1' in the number in Contacts. The second time, I added the '1' back. Doesn't make sense to me, but it worked.