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Not all mutations are advantages, some are disadvantages but aren't severe enough to hinder the species, or they develop other mutations that make up for those negative mutations.
Evolution is random, it favors outcomes that have high fitness values but does not give everyone the highest fitness value possible. Change happens when there is pressure to change and specialize.
As for this monkey in particular, he may have a better sense of smell which is not a great advantage, but useful and having a big nose doesn't make you that much more of a target/easy prey.
It's likely that descendants exists that have lost that specific trait