I recently purchased a Hoya RM-72 IR filter to experiment with. I like the effects I can get after post processing, however, I have noticed a reflection in the very center of the shots with it. I'll post an example below. The reflection disappears for the most part once I do my post processing, but I'm still not quite sure what's causing it. The camera gear is a 6D with Sigma 35 f/1.4 Art, and the shot settings are ISO 800, f/8, 25s. I haven't been able to see if it appears on shorter exposures as I haven't been able to go much lower than a 20 second exposure at noon on a sunny, cloudless day.
The circle reflection is visible in the center of the first attachment. It is in the same spot and has the same intensity regardless of the position of the sun, cloud cover (direct vs indirect light), and it almost always clears up after post processing. Does anyone with experience with this filter have any input on what it is and how to mitigate it?