If we create a new issue from the board, we can only add the title:
But even when you open the form of that issue, it is limited in what you can do:
So, as you can see above, it does not contain all the useful features of the form (below) which opens when you start the new issue elsewhere:
I wonder, why there are two different forms for creating a new issue? It would be simpler (and much more useful) if, when we click “New Issue” from anywhere, the complete version of the form would be made available.