Just an Update,
I had contacted to MATWORKS Support and I shared this problem with them as well. To my surprise this is a error and we can't plot legends on figure using axis handle only. They will try to work on this error and get updates on next release may be.
But I found a workaround soultion from one of the similar questions asked. Here is the link to that question. How can I save UIAxes as an image when button is pushed in app designer? - MATLAB Answers - MATLAB Central (mathworks.com)
The solution is that the person who answered to this question has created a function called as copyUIAxes(). This solution will only work for the Axes coming from GUI as UIAxes having 'matlab.ui.control.UIAxes' class. This will create a struct containing axes, figure, legend classes within.
For me this solution has worked but lets hope we can get update from mathworks as well on future release.