This problem occurs because you hold B to end your turn, but when you release it, it is no longer your turn. Since almost all in-game actions occur when you release a button instead of when you press it, the game will see you releasing the button after your turn has ended and apply this action (B) to the start of your next turn, which will make you come out of sneak.
You can easily get around this by holding B to end your turn, then, any time after your turn is over, holding RT to bring up the selection wheel, then releasing B. The selection wheel will come up, but releasing B will cancel out of it, so the game won't keep that B action until your next turn.
I'm having to deal with this a lot because I'm doing a playthrough with 2 Lone Wolf crossbow users, both with Sneak 5. I end every single turn in sneak mode, with both characters.
Just hold B, hold RT, then release B. At least until the patch comes.
Last edited by CharityDiary; 20/05/16 03:37 AM.