I am encountering a very annoying bug with the Pivot Table in QlikSense, where randomly the cell appear blank, they are not empty or null, because if you hover over any cell you see its value in the tool-tip.
Happens very frequently, to the point of making the visualization unusable. Some times closing and relaunching fixes it, some times reloading the data, or adding/removing the dimension.
In the problem I saw, we were using a background color expression. It finally "went away" after I re-created the chart from scratch. As Erik suggested, it might be helpful to check in with support on this.