base30 getData is empty

[ninescore]

1、$("#signature").jSignature("setData", 'data:image/png;base64,iVBORw0K......
2、The picture is displayed normally
3、$("#signature").jSignature("getData", 'base30')
4、console print result：['image/jsignature;base30', '']

Loading existing base64 can display, but can not output base30. If it is drawn manually, it can output base30. If you both load base64 and draw manually, when outputting base30, only the manually drawn part is included. why is that?