Have you copy and paste the Artist name to be 100% sure there is no extra space or other not obvios difference?
yeah, 100% sure. i tried adding a space after the name in Album Artist, then they will all remain as one artist; once removed the space, ta da... back to two artists with exact same name same spelling same everything.
on a side note, this is the 2nd time it happened to me. the first time, what i did was adding a space after the name, clicked OK; then removed the space, clicked OK again. it solved my problem. but this time, doing the same thing won't solve my issue.
don't know if it means anything, but both the artists have more than 33 Albums in my iTunes.