MichaelChen: Ok, I wouldn't say definitely that the mutagens disappear or not, but this is what happened to me. My initial armor was 112 before respec and I have all mutagen in alchemy route and swordsmanship route. After respec, it did not change any of my swordsmanship skills, but put the points from alchemy to magic. check armor, still 112, and check skill description, the mutagens were still present when you mouse-over the skills. So, I thought, SWEET!!! But after a while I am not quite sure what triggered it, (lvl up? equipment change?) my armor value dropped to 90+, and I rechecked the skill description, the mutagens in the alchemy trees were gone. So, hoped that helped.
On a slightly different note, since we are talking about expoits. Did you know there is an exploit in which you can get the respec and the drops from the Overseer (was that the name?). You must know the throw dagger skill, and bring a lot of daggers (around 50 to be safe). You enter the room where the boss is, but don't go down the stairs, and start throwing daggers at him, use quen or retreat up the stairs when he starts fire-balling you. You will need a little luck, as he may heal and stuff, but its doable. You can kill him before initializing dialogue, and then go down the stairs. The dialogue for respecing will trigger and you get the drop from his dead twin. You can actually kill him twice and get two drops, but I don't see why you would want to do that.
Nice exploit. I never thought of talking to the Operator after cheap-killing him.
As to OP's question, I never really paid attention to it but logically I expect the mutagens to disappear lest you get 2x the mutagen effects (that is, if you have enough mutagens).
I am glad you raised the question though. I never thought about the mutagens before.