I agree that the relationship with myopia is likely a coincidence.

Blood sugar variations can cause significant myopia but it is transient,
usually resolving after the blood sugar is controlled. Surely they've tested
for that.

The most common reason for permanent myopic shift at age 57 is a change in
the crystalline lens that occurs preceding cataract. It might not be visible
in the examination, or may only appear as a mild yellowing in the lens. It
can still take years for a significant cataract to form.

-MT, OD

I'd agree with Dr. MT's assessment, although it is theoretically
possible for Horner's to cause a random and significant increase in
accommodation, which might appear as increased myopia. I don't see a
model where the myopia would be constant in power over a long period of
time. Never seen such a thing, but it's possible.  And easy to
differentially diagnose by doing a cycloplegic refraction.

Assuming no acute blood sugar issues, the place we look first is the
crystalline lens.  I had a patient with a 6 month progression of myopia
to - 2.00 with 3D of astigmatism, not fully correctable with specs. The
diagnosis was anterior lenticonus, a somewhat rare condition where the
anterior of the lens becomes steeper and irregular. It was followed by
the same condition occurring on the posterior lens cortex and finally by
cataract about two years later, and fixed by IOL surgery. Diffucult, but
not impossible to see with the slit lamp biomicroscope.

--LB, O.D.