Hi WLCSoft,
Thanks for reporting the problem.
To use the merging feature of this extension, the sprite collision mask must be set to "Use full image as collision mask". This is not the default collision mask for sprites. (The default is "automatic collision mask".)
If the merging feature does not work, change the sprite collision mask to a custom collision mask and then select "Use full image as collision mask".
Note that this issue does not occur with Tiled Sprite objects.