I have had this happen before too, but don't remember the fix.
Try this though if you get a chance. Increase Micropoly detail to .8 or even 1 and see if the seam goes away.
Also try increasing your GI settings to see if there is any effect. Try 3, 3, 8 or 4, 4, 16 (Default is 2, 2,
.
Also does the seam happen at all resolutions? Try rendering 4000x2000 or 8000x4000 and see if the seam is there.
If you have a test scene share it here and I'll test it on my machine.
EDIT: I did find this in the Docs:
"GI prepass padding: This specifes how much to enlarge the area of image calculated during the prepass, which may be useful to avoid mismatched lighting between adjacent cropped renders or pieces of a panorama when not using GI cache files (GI cache files effectively solve this problem). A value of 0 means no padding. A value of 1 adds a border to each side of the frustum which is equivalent to the width or height of the image or crop region, which makes the frustum 3 times as wide and 3 times as tall. Negative values shrink the area/frustum instead of enlarging it."
Anyways, I seem to remember it had something do do with some of these settings.
Derek