I play Pokemon Go and I have evolved 15 Eevees and they are all Jolteons. I wonder if this is a glitch or a Pokemon Go feature. I have known about the Name Trick, but I don't like using hacks or cheats to get something special. Is it that the first evolution you evolve for Eevee is the permanent evolution except you use the Name Trick?
Just chance. One in 4,782,969 players would have their first 15 regularly evolved Eevees become the same Eeveelution. Given that the game has far more players than that, it's actually fairly likely to happen to someone, you're just one of the very unlucky people who have had it happen to you specifically.
I have known about the Name Trick, but I don't like using hacks or cheats to get something special
The name trick is neither, but it's best saved for use on shinies or hundos anyway since you can only use it once per Eeveelution.