Modern technology has no problems creating a device that floats thanks to LTA gas or is assisted by it. Evolution has created systems in animals that are even beyond our current level of technology, even very complex constructs have been found. I.E. the chameleon which uses nano-crystals to archive it's color change, something that could be very beneficial for modern displays once it is fully understood. There is no reason to believe that even a complex system to fly using LTA gas is impossible.
Evolution does create things "randomly", but if it turns out to be a disadvantage for the species it literally dies out, if it is an advantage, it will strife, if it is neither it might continue to exist in one form or another. So if LTA gas is something that is an essential part of a species, it has to provide some benefit to this species. There might also be species that have a 'left-over' version of LTA gas flight, that does not put them at a disadvantage, but doesn't necessarily provide any noticeably benefit either.
The amount of LTA gas required to cancel out gravity is enormous. Compare the size of the man in the picture to the volume of helium necessary to lift him up and consider that the amount of gas necessary to lift an even bigger species raised exponentially. Getting an animal the size of an elephant to fly with balloons would require so many, that the elephant would look tiny in comparison.
Any larger species that uses LTA gas as their main system of lift would have to look massively bloated. As friction raises with the surface area (drag), such a species would be either very slow, or it would have to have a body that would reduce friction. A flying squid would barely be able to fight the force caused by wind on his body, a flying Manta could however be very maneuverable. Water-based creatures serve as good examples here, as they have to fight the same issues at a smaller level.
Further issues caused by LTA gas like flammability, leaking, producing gas reserves are no relevant issue, because if such a species would exist, it would have already been solved in a clever manner, otherwise they would have died out at some point and it wouldn't exist. Gas sacks would be sufficiently covered by dense tissue to prevent leaks from predator attacks, and even rare gasses could be created by that species organism at a sufficient rate. There are bacteria that turn iron into gold, so why shouldn't there be an organism that somehow produces helium?
So why are there no LTA gas flying species around here?
The main reason would be that evolution favors simpler creatures, as they can adapt to environmental changes easier than complex creatures that require a very specific Eco-system to exist. Additionally mutations do not happen instantly to something completely random, but gradually, so there had to be a base creature with an air bladder that slowly evolved step by step into a flying creature with LTA gas, which may simply have never happened, or the creature died out on the way.
Huge flying creatures that would benefit from the lift of LTA gas bladders could have evolved, but such a species would require more food thanks to it's size, and therefore it would be more dependent on a vivid Eco-system to get that food. A simple drought could have killed such a species or it's ancestors before it became widely spread, similar to how dinosaurs died out from a sudden change to the Eco-system.
Summary
A species flying with the help or entirely based on LTA gas bladders is possible, but unlikely to evolve by natural cause on this planet, as it is complicated yet provides no substantial benefit over simpler alternatives. This could be entirely different in entirely different Eco-systems, or with magically accelerated evolution. So a creature flying thanks to gas bladders is entirely possible in a different world, but not necessary.
