1. new worksheet, fill XYZ with random numbers, make contour plot

2. open PD dialog, go to label tab, set Numeric Format to Scientific 10^3

3. go to Colormap/Contour tab, click Labels header, select Show on Major Levels only

4. click OK
   ==> Message Log: Undefined variable: X00D7

5. make contour plot

6. open PD dialog, go to label tab, select custom as format, click OK

7. reopen PD dialog , go to label tab
   ==> no edit box for custom

Fixed in Origin2021b.