|...except that it's not happening to me.|
Of course, I don't have your data, your system, etc.
Here is something to try:
If you run the code below, it will fill your ranges with hard coded numbers as a test.
Vert_Braid will contain sequential numbers from 1 to 2583 (the number of cells in the range).
Vert_Load will contain random numbers between 1 and 2583.
NOTE: Neither of the ranges I tested this on have any merged cells. I'll leave you to play with this and make the necessary changes to have it work for your merged cells.
After you set up the ranges via this code and then run your macro, it should be fairly easy to tell if the glitch occurs with this data or just yours or just in your workbook (corruption?) or just on your system, etc. When I set up a worksheet in this manner, I see no difference in which cells are colored by the code at any zoom level.
'Fill Vert_Braid with sequential numbers
For cellVal = 1 To 2583
Range("Vert_Braid").Cells(cellVal) = cellVal
'Fill Vert_Load with random numbers
Range("Vert_Load").Cells.Formula = "=RANDBETWEEN(1,2583)"
Range("Vert_Load").Cells.Value = Range("Vert_Load").Cells.Value
'Color each column of Vert_Load
For colNum = 1 To 8
Range("Vert_Load").Columns(colNum).Interior.ColorIndex = colNum + 2
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