Avoiding a particular job is not the same as sitting around doing nothing. It's quite possible for you to get basic income, yet channel your energy to other productive things.
One of the problems of those studies are that they are temporal, and everyone involved know it. If I enrole you for two years in an UBI experiment, you'll probably not leave your job, even if you hate it. But if I guarantee you the same income for the rest of your life... well, lets take a look at how many people keep their jobs, and specially their boring/shitty jobs, when they win a huge lottery price. Usually they don't do other productive things, or even other improductive things: they tend to destroy their own lifes.
Another problem is that you have to take into account how slowly people internalize the new situations. E.g. when central banks started to use inflation as a source of government income, in the first decades it was "free". But people adapted and started to track inflation and price indexes to peg the salaries to them, "learning" how to live with the new situation.
enrolling in a study is not at all representative of how people would behave if the same were a permanent
I've been following the UBI space for a long time and I've never seen anyone use this as an example - that's just a strawman. What people discuss is the work that is valuable to society but is unpaid such as raising children, caretaking for the elderly, community support, and so on.
A few hundred dollars a month isn't enough to live on, but it moves the needle for those most at-risk for going hungry or becoming homeless. And for everyone else, it gives extra breathing room and negotiating power in the workplace.
What you talk about is not UBI, but other schema of basic income conditioned (thus not fulfilling the U).