2nd Fighting Style (lvl10) removes the 1st. The only way to keep the first is to go to the screen to choose the second, then go to previous screen and then go forward again to the choice screen. They will all then be there, so you can choose the one you had 1st. But doing this means you never get the second one that you should have.
I've tested this using the fighter. I'm unsure if this remains the case for any other class.
Also, this is using game v1.0.22 Final (from GoG)
Oh, so THAT'S why my fighter stopped blocking with her reaction. I can confirm I have the same bug.
Bump, also experienced this issue with my fighter.
Yep, just noticed it with my 10th Level Fighter, too.
Got Great Weapon Mastery with his first Fighting Style and took the AC+1 thingy at level 10.
He no longer gains the rerolls on damage die when rolling a 1 or 2. Strangely enough, he has kept his bonus attack
Link to screenshot as I can't upload it here strangely: https://ibb.co/7Q1jXMk
I must say, this is really a big one for me, as it decapitates my main damage dealer for the final part of the game (which I'm really enjoying very much so far :)