Yes, it's been like this for a little while now. You have 30 days after you ask a question to pick an MHO, if not then the system picks it for you. And yes, whether you or the system chooses the MHO, it cannot be changed afterwards.
It's set up like this in order to encourage question askers to choose the MHO themselves. It earns both the QA and the answerer Xper points, and of course it shows them that you thought they had the most helpful or interesting response. Plus you'll actually get to decide which answer you want as MHO, unlike sometimes when the system happens to pick one you might not agree with as being the most helpful.
Part of posting a question is choosing a helpful opinion afterwards. It shows appreciation to all the people who put in effort and gave you answers. The system is done that way to encourage people to select a most helpful opinion as well. If you don't want one being chosen for you then the solution is simple, select your own one :)
It bases the choice on the users Xper rating, how many comments and likes it got so it's not completely random. I think it's good, but neither of you get Xper for it. It helps me because some questions, there is no right or wrong answer and you're not asking for help, just views so you can't choose one! It's good the site chooses and someone gets MHO. Some people pop on, get help and never come back so at least someone can get MHO.
I dont like it, so I chose my MHO before it reach 30 days
I actually think it's good. After 30 days it's highly unlikely that a question will receive new answers, and it's also unlikely that people who asked the question will go back after 30 days to pick a MHO. I am not bothered by it in the least. I simply pick a MHO after a few days to 1. beat the system and 2. show that I appreciate the help I've received.