Hi Lachlan,
I suggest you check the following:
1. The cells are merged, you can unmerge the cells.
2. The keyboard has a sticky Shift key, which will result in a selection of a range of cells.
3. The extend mode is currently turned on, you can turn it off by pressing Fn+F8.
4. Make sure you are in Normal View.
5. Try altering the zoom level.
If the issue persists, I would like to confirm the below.
1. A screenshot about the version of Excel.
Click Excel> About Excel.
2. A full screenshot about the highlighted multiple cells.
Regards,
Yoga