Leaving and re-entering makes age disappear
When leaving the app either via home-button or via multitasking and going back, the age of one contact born on the 31st of some months is gone. Repeating the process will hide the age of another affected contact. Some birthdates aren't affected I was not able to determine the reason.
I attached a demo contact 100% affected. vcards_20200325_090519.vcf