Dark spots in corners of images for 360 panoramas

I ran SNS-HDR on the 3x bracket sets before stitching panorama in PTGui. I can't find a good way to get rid of the dark spot. Does anyone have another solution? I have tried running SNS-HDR in both 360 mode and regular, but doesn't make much difference, and also the PTGui "Automatic Exposure" setting, which also doesn't seem to make much difference.
SNS-HDR is making the corners darker of each bracket set. Is there any way to fix this?
SNS-HDR is making the corners darker of each bracket set. Is there any way to fix this?