It could, but it might also lead to something harmful for the deer at the same time. I’m not sure if the gene affecting the deer’s eyesight is known, but it could be a pleiotropic gene (a gene that influences multiple traits at once).

If that’s the case, and the other effect is negative and somehow spreads through the population, it could become a future issue for the deer. Think about humans—we lost the ability to produce our own vitamin C. Almost every other mammal can produce their own (except for hamsters). When this happened, it didn’t harm us right away, so it spread through the population. But over time, it led to issues that weren’t a problem before, like scurvy.

Same could happen to the deer.