I've done autosomal DNA tests on ftdna for myself, my mother and paternal grandmother. I've transferred the results to MyHeritage, where I have a match with a person X with the largest segment of 27cM. Neither my mother nor grandmother match with X, so I assume it has to be a match from my paternal grandfather whom I have not tested. Looking at the match’s ancestory I can indeed identify common ancestors with my paternal grandfather. However, looking at the matching segment's location on the chromosome map, I see that it falls in an area where I also have a match with my paternal grandmother, which means that I cannot at the same time also have segments from my paternal grandfather in that same location. There's a clear contradiction there. Should I regard it as false positive or is some other explanation possible?
Edit: the following image is a screen capture from MyHeritage's chromosome browser. In red are my matching segments with X (6,3cM in cr 18, 7,9cM in cr 20 and 27,2cM in cr 22), in orange my matching segments with my paternal grandmother. As said, X and my paternal grandmother do not triangulate with each other. As seen, all of these three chromosomes (18, 20 and 22) are entirely from my paternal grandmother (the other pair of the same chromosome being from my mother), therefore there cannot also be any DNA from my paternal grandfather in the same chromosomes.
Update: The matching segment in chromosome 22 has a triangulation group consisting of possibly 10+ persons, including me, my paternal grandmother and X. Everyone triangulates with everyone, except my paternal grandmother and X, who both triangulate everyone else in the group, but not each other.