Have you ever had a fitting issues that you could not find addressed anywhere? I have one of those and it is in the sleeve cap. Below is an example and you might need to zoom in a bit:
I finally found an example in one of my fitting books and it was in The Minott Method - Total Pattern Fit, This particular book is specific to dresses, tops, and jackets. I also purchased the Sleeve Package from Sewing Artistry.
The Minott Method calls this Sleeve Pitch while Sewing Artistry calls this sleeve tilt. Both show examples that look just like the back of sleeve head on the muslin above. However, different solutions are given. The Minott Method gives the solution at the pattern stage and Sewing Artistry gives the solution at the fitting stage.
I doubt that the muslin above is still hanging around but I can test the Sewing Artistry method on the next muslin that I make and use the Minott Method to correct the pattern. This sounds like a plan to me.
Now just to be clear, I see this issue with just about everything I make not just jackets. I guess this means I need more length at the back of the armscye in the sleeve.
Happy Stitching!