I don't know about a diagnosis code, but I did have an MRI in 2019. The only thing it showed was some white patches probably due to migraines (the patches in the brain seen in migraine patients look somewhat similar to those seen in Alzheimer's patients, but I'm told "not to worry"). I have been having migraines since my teenage years so it's not clear whether it's relevant to the photophobia or not, but after visiting multiple eye doctors who could find nothing physically wrong with my eyes, their best collective guess (among my primary care doctor, the ophthalmologist, and the neurologist) was that the flickering triggered a migraine that localized the pain immediately in my right eye with persisting pain throughout my head in the weeks afterward.
It's only one possible explanation, rather than a conclusive diagnosis, but at least the MRI rules out some of the other potential neurological issues.