When exporting images with 'Add to Stack' checked only one (1) image file can be stacked with the original. When a second export file is created using the same original file it is stacked with the original, but the first export file is removed from the stack. The first export file is then moved to the end at the bottom of the Grid view (last image). Changing Sort Order to Capture Time or File Name or A>Z, Z>A has no effect. The file remains at the bottom position of Grid View as the last picture. I also reset the LR Classic Preferences file with no change in behavior.
If you right-click on the original file and select 'Unstack' all of the files appear together as expected using Capture Time sort in the Grid view. I checked LR CC 2015.12 using the same image file in the same folder structure and it behaves as expected with all files stacked below the original.
Both Windows and Mac OS X platforms exhibit this issue with LR Classic 7.0 and 7.0.1. Please see this post for more details: