After Effects Blur Effect with Mask Doesn't Go to the Edge
If you applied the blur to an adjustment layer (as you should), check to make sure the layer is positioned in the center of the composition. Hit 'P' on the adjustment layer and right click on the 'Position' name and Reset it.

Check the mask expansion to see if maybe you have it set to a number less than or greater than 0. I doubt this is the issue as when you recreate a mask the problem doesn't persist.

If you haven't applied the blur to an adjustment layer you should do that and see if it fixes your issue.