I've never heard that before. It was always my understanding that Behind The Mask never made it to Westlake, and that it was never revisited after the Thriller sessions began.
I doubt Michael would have wanted to release his version after Greg Phillinganes and Eric Clapton had released theirs.
Furthermore, Behind The Mask was not written on Michael's to-do list in his bedroom, which contained songs he wanted to work on for his next album. I know the Estate has written that Michael worked on the song for a long time. However, that could be a justification for their decision to release it on Michael in 2010, when the whole idea was to release Michael's last songs, not some demos from the 1980s.