You can determine the version of JMP you are running by going to
Help==>About JMP
I tried to change the legend setting in both JMP 13 and JMP 14, and both showed the same issue that you discovered. The Chart Platform has been devalued in JMP 14. Therefore, I doubt if you will be able to get a fix from the JMP folks. However, you can duplicate the chart you want in Chart Builder, and as I indicated in my last response, you can change the legend settings in Graph Builder.
Jim