If a human dies after obtaining a Jhana(and preserved it) he definitly gets his rebirth at a Brahma world.
So, what makes a Brahma come back to human world upon death? Is it possible that he was unable to preserve the Jhana?
Everything is impermanent. What the Brahma enjoys is the Pala or fruit. The fruit of any Karma including that arising from developing the Jhanas also passes after sometime. If you develop the Jhanas they also have a finite duration which you can have them before you loose them. The duration can vary based on past pratice, but as always finite. As in many cases if you lose the Jhana before the point of death then you will not be able to go the Brahma world as you need to have the Jhanas at the point off death for it to bear fruit.
"he definitly gets his rebirth at a Brahma world" I would not speculate such in a linear manner. It's not sure when fruits are actually ripen. As Upasaka Suminda Sirinath S. Dharmasena mentioned, it would need to be your passing awarness.
In regard of why does one not stay at a level of merits: You could see it like having earned a lot of money. Once you have it, you dwell in a pleasant adobe. Yet in this adobe, you have no possibility to earn money at all. So its secure a matter of time, that your earnings will go to an end. That's why it is not seldom to even fall into very low levels of existence, till such a precious live like a human could be gained again, which is not for sure. That is why the Buddha always recommended to gain stream entry and lesser seek for a certain not lasting existence, how pleasant it ever might be.
Some other aspects and how it happens when Devas die see Iti §83