As long as the fitness change isn't too big I'd say things are going as to be expected.
The "Keep the best creatures" options means that the next generation will always contain the chromosome of the best performing creature of the previous generation without any crossover or mutation. Because the creature movements are based on many different factors, it is very hard (if not impossible) to guarantee that the same creature with the same brain will behave identically to its last run.
Even very small changes in one of the inputs or outputs can sometimes cause a fairly big difference in the behaviour you see. The difference between landing a jump in a recoverable and smooth way and tipping over can sometimes be the difference between extending one of the muscles to 53% or 54%. This is also why you can watch a creature that has a seemingly perfect strategy of moving its limbs in a steady and repeating pattern trip up after some time before then recovering and going back to its routine. Sometimes they also can't recover a little fall like this because their brains are only optimized to work with that one steady pattern. If anything slightly changes, the brain might not be able to react to it properly and instead only cause everything to get worse.
Your creature might have gotten lucky in the previous run or it might have simply gotten unlucky in the following run. Either way, as long as the performance doesn't just rapidly decrease by a lot, things don't seem to be broken.
By the way, a fitness of 20% in generation 5 is pretty great if you ask me. A running fitness of 100% is ridiculously difficult to reach, as long as you don't use the glitch of connecting multiple body sections only through muscles instead of bones (And reaching 100% with a glitch really shouldn't be anything to get excited about)