Mine is sorta like that, too.
It doesn't happen constantly because on some sides of the tank, he can't see his reflection.
I'm certainly no expert, but it seems like it could be some cause for concern if it really happens for hours a day. But I don't think the flaring itself is going to hurt him. Just maybe means he is on edge for some reason or maybe sees something that is scaring him other than his reflection?
Quasi-funny anecdote: I had the container of fish food set up next to one area he can see through, not even realizing that the betta on the bottle was facing him, and I noticed him looking at it and flaring a little. Oops!