I just finished a charity ride and found that the brand new jersey bought for it has a hole in the back center pocket. It is neatly cut, with stitching around it to prevent fraying, so I'm sure it is a feature rather than a defect. What is the intended purpose?
My best guess is the hole would allow a headphone cable to run down the collar, through the inside of the shirt, and into the pocket.
I'm going to guess the garmet manufacturer puts the hole there so that when you wash the jersey (which hopefully you do occasionaly), water doesn't pool in the pocket but instead drains out the hole.
This doesn't really explain why it's only in the center pocket and not the side ones ...
I remember that decades ago when wool cycling jerseys were common, the middle pocket in the back sometimes had a button and button hole. You could button the largest pocket. I don't remember if the other pockets also had buttons and button holes. My guess is that the stitched hole is a vestigial button hole from an old jersey style.
For some sort of cord to make life easier.