I can't shed any light on why this is happening, but I do have a suggestion for a work around. There is a free app that not only lists your birthdays in your google calendar, but also shows their age. It's free and works for me. After installing, go to Google calendar settings where you choose which calendars to show. Uncheck the stock Google birthday calendar and then check the Birthday Adaptor calendar. Maybe this will work.
Birthday Adaptor
This looks good but the birthdays are showing up twice. How do I fix it. Thanks so much for this information. I appreciate it.
Samsung Galaxy S5 Lollipop 5.0 Verizon network
Is one showing up with the age and one without the age? If so, one is from Birthday Adaptor and the other is the stock Google birthday. If that is the case, you need to disable the Google birthdays. Open Google calendar. Press the menu bars (upper left hand corner) and scroll down until you see the listed calendars. On my phone, The Google birthdays is listed under "More" with "Holidays" under it. Remove the check mark.View attachment 201434
Is this on the actual Google Calendar app, or the Samsung Calendar app? On Google Calendar, if I select an event, then tap the Menu button, the Delete option is there.
It could be that the ones not showing up in calendar are not actually saved to your google contacts, but are instead saved directly to the phone or SIM.
Did you verify that is the problem. It not, open one of the contacts that does not show up on your calendar. You should see "Connected via" and there will be a symbol there to indicate where it is stored. If it is Google, you will see a "G". If it is stored on your phone you will see a phone icon. To transfer contacts from phone to Google, follow this link for instructions.
samsung galaxy s - Transfer contacts from android to google contacts - Android Enthusiasts Stack Exchange