1) It's main solar effect seems like a watered-down version of SCP-1008. It's interesting that the fisherman experience physiological consequences from moonlight (although I'm not clear on how that happens, more below), although attributing physical maladies to it… also leans back towards 1008. I think the historical angle is interesting enough to lend this some weight, but it's still mighty similar.
Also,
altering the sensory perception of the Moon and Sun for individuals who have observed SCP-2248-A or an image of it (hereon referred to as subjects).
Seems redundant, given that you immediately describe those effects.
2) Similarly, the description of the blinding of the fishermen seemed like a repetition of the section on moonlight, although I'm very unclear on how they were blinded. I mean, if moonlight essentially becomes as effective as sunlight for them, wouldn't their reflexes to occasionally shade their eyes, blink, or not stare at the Sun/Moon just adjust? I mean, those are generally automatic, physiological responses.
3) Tense issue:
When initially discovered, the plant species… are found in SCP-2248. Seeds… are found as well,
"When initially discovered" indicating a past event, followed by current-tense verbs. Drove me batty.
4)
SCP-2248-A is classified as a Type DV cognitohazard
I'm all for crafting new terminology, but when I scrolled down to see that it just meant "Direct Visual," it felt like a waste of my effort. It seemed like something that didn't need to be summarized, like a useless bureaucratic gesture rather than a helpful designation for a complex effect.
This isn't something like Type 2 Poltergeist, which suggests a vast expertise on something the author doesn't need to expound. I'd be more comfortable with something like IINDV Cognitohazard for "Immediate Isolated Non-reversible Direct Visual." Just some kind of demonstration that you're really packing in the information, rather than shifting half a sentence about photography into a footnote.
5) Positive point, I can imagine all kinds of narratives arising from this, which is great. Legionary and priestly adherents of Sol Invictus dragged before the statue to deprive them of their God. Members of the cult stealthily overtaking Legion positions at night, only to be slaughtered wholesale in their blinded condition in the following daylight. Neat stuff.