DO NOT USE MERGE
You have found out how limited merge is and how it can and will screw up your work sheets.
Avoid merging cells
Merged cells can help you arrange values in a meaningful way, but they come with problems -- numerous problems, big problems.
For instance, Excel won't apply column formats to a merged cell unless you select all the columns that comprise the merge.
In addition, not all cell formats, stick once you emerge a cell.
You can't sort a column with merged cells.
You can't even select a single-column range if there's a merged cell in it -- go ahead, try!, the whole column will become merged, not good.
You cannot put a filter on it. The problem is the filter is completely useless because the filter will groan with the "merged cells need to be identically sized." Warning, which in English means you have to make each group of merged cells the same size as the largest group. And you have to find them all!
Merging cells in columns and rows could lead to data loss, bad thing.
Formulas and Functions that refer to merged cells will not work, bad thing.
Don't hesitate to use merged cells if you really need them (you don’t), but they will limit what you can do to the cells and even the columns involved.
Center Across Selection is a far better alternative to merging.
To apply this format, select the cells you want to appear merged and then launch the Alignment group dialog, Ctrl + 1, and click the Alignment tab. Center Across Selection is in the Horizontal drop-down.
You will get the desired look you want but without the merged cell's problems.