I have an ongoing issue...various spreadsheets (none in particular...could be
one or two or all of them) will start taking on this behavior for no apparent
reason: when you click an individual cell, it highlights that cell plus
usually two to the right of it. Reformatting by using the format painter from
an unaffected cell (if you can find one at that point that hasn't been
affected) usually corrects it but it's time-consuming to manually go back and
reformat each affected cell. What causes this behavior and how can I stop it?
Or is there an easier fix for it? This is affecting multiple users within
our company.

Have found related posts @ Microsoft, but nothing definitive that MS
apparently has not yet acknowledged that this is, in fact, an "issue".

My suggested workarounds are as follows:
1. Try tapping the wheel of scroll mouse, if applicable.
2. Try changing the page view from Page Layout to any other View option:
View tab Normal
3. Close the spreadsheet, open again and see if behavior still exists.
4. Close Excel, open the spreadsheet and see if behavior still exists.
5. If cell still selects multiple cells, up keyboard up/down/left/right
arrows to navigate away and then back to cell, then click in the formula bar
to edit
6. If it says EXT or Extend Selection in the bottom left corner of Excel,
then it is in Extended mode - try tapping the F8 key once
7. Try zooming out then clicking the cell selection again: View tab Zoom
Edit % in Custom field Click OK

TIA!