Mount ceiling fan to joist on sloped ceiling
I would consider getting a metal plate adapter similar to this:



You can mount this to the screw holes in the ceiling electrical box. Then in turn you can mount the fan hanger bracket in the correct orientation. As shown you can see there are plenty of options to adjust the positions of everything including the possibility to install captive bolts and nuts to which you could attach the fan bracket.

Edit:
There are various places to acquire plates like this. Google search for "ceiling fixture adapter plate" and you may locate a source such as here:

https://www.platt.com/platt-electric-supply/Metallic-Box-Accessories-Mounting-Brackets-Universal/Dottie/SP34/product.aspx?zpid=674218