I have tried to reproduce this issue but failed. It could be a has a chance of theme or plugin conflicts. You can disable all your plugins except Dokan and switch to a default theme of WordPress like – Twenty Twelve, Twenty Thirteen, Twenty Fourteen and let me know the results.
I did as suggested, I deactivated all plugins except Dokan and switched to a default theme, Twenty Fourteen. There was no problem.
But when I activated all the plugins again one-by-one, there was still no problem. Until I switched to Dokan theme and the problem showed again. The conclusion is that the problem might be in the Dokan theme.