If you modify the number of time points, you must modify also the Time vector.
To get a correct file template: select the original file, and run the process "Average > Average time". It will create a file with two time points: the beginning and the end of the time segment.
This convention in Brainstorm helps representing a single value that has a temporal extent. To follow the same convention: simply duplicate the ImageGridAmp matrix: ImageGridAmp = [ImageGridAmp, ImageGridAmp];