Thanks for pointing it out.
I observed this issue as you mentioned, Aspose.Cells.GridDesktop does not behave similar to what MS Excel does. When we copy/paste a merged cell to some other location in the worksheet, the destination cell is not a merged one where it should be. I have logged a ticket with an id “CELLSNET-44802” for your issue. We will look into it soon.
Once we have an update on it, we will let you know here.