Step to reproduce:

1- Enable (org-indent-mode) in an org buffer.

2- Use the command (org-inlinetask-insert-task) 

3- Insert some text between the task's title and the keyword END

4- Notice that the text between the title and the keyword END isn't properly indented. 

I'm not sure if this is helpful, but I noticed that we have the right indentation when using the code from org-inlinetask.el of this old repository:
https://github.com/ngz/org-mode-lists/blob/master/lisp/org-inlinetask.el

Thanks for your work, org-mode is an amazing piece of software.