Mine has that issue, even with bright room light and such. He flares at anything that causes even the slightest reflection. The thought that flaring causes stress or is bad for males is a fairly new one, back when I first started keeping betta I don't recall reading that notion in printed or online literature regarding betta splendens. So unless the fish is showing signs of stress like hiding, listlessness, not eating, washed out coloration, or clear agitation beyond the normal posturing, I wouldn't worry about it.
Some males are more aggressive and some are more mellow. The more mellow males are the ones I worry about, as they seem more prone to agitation and stress regarding the male they think they see. So unless you are noticing ill health effects with the reflective flaring, just enjoy the display and ignore it. Some males stop doing it after awhile, some (like mine) do not, but so long as the fish is healthy and displaying normal behavior (which posturing and flaring definitely is!) I don't sweat it.