A very reliable form of mutation I might add, that actually formed alleles instead of mistakes, unfunctioning alleles or diseases, a mutation rather different from that we know today.
But that kind of mutation, as you point out, doesn't exist. Never has existed. Is not necessary at all.
As has been noted over and over. Each human is born with a number of novel mutations. In addition, a large number of humans are not born because they started off with malfunctioning mutations and spontaneously aborted. Roughly half of the "tries" are removed this way. (more or less since many of those abortions may have been for non genetic problems).
In a population of 100,000 reindeer each year there maybe 30,000 new births. Already tens of thousands of "tries" have been removed. Over the next year another 10 or 15 thousand young reindeer will die. Some of these had "unreliable" mutations.
You don't need "reliable" mutations. You need enough "okish" mutations out of all of them and a mechanisms for getting rid of the "unreliable" ones.
When we examine the real world that is what we see.