Yes, it is most likely devitrification. Devitrification does not have to be grainy, It can look just slightly dirty. Darker glasses show devitrification more easily than light coloured ones.
If you are grinding the edges after cutting the strips, this will promote devitrifrication. If you are placing the strips on edge, you do not need to grind them at all. The fusing process will deal with the little cutting imperfections, and by leaving the cut edges “clean cut” there are no little pits for the devit to start.