I hear your eyes use up about 25% of your daily energy. When you fast your energy source and levels change. That could be why. I do eye exercises because I’m weird. Sometimes I wear an eye patch to try and combat my lazy eye. It helps, but I don’t know if I’ll ever get 20/20 vision. I hear extended fast improve your vision though. I hope I helped.
Fasting can also be helpful in bringing underlying problems to light as well. Might be worth having an eye exam if you haven’t in a while to rule out other things. Might also be related to the release of toxins that fasting can induce. All theories here though.