.
Regarding Roger Stern and John Byrne's run in CAPTAIN AMERICA 247-255, I knew at the time that Stern and Byrne had both left the book abruptly, for reasons not disclosed at the time.
This link on Byrne's site answers that question :

https://www.byrnerobotics.com/forum/forum_posts.asp?TID=39411

Stern says it was for deadline reasons, and being treated badly by book editor Jim Salicrup.

Byrne has a different recollection that it was Jim Shooter's sudden order that all stories had to be resolved in a single issue, that Stern wanted to comply after completing a 3-part Red Skull storyline he had just plotted. That Shooter insisted had to be condensed to one issue, that Stern resigned in protest, and Byrne resigned in solidarity with Stern.

Byrn'e version rings more true to me, of all the factors involved. Stern's version makes Jim Salicrup look like the bad guy. Byrne's version makes Jim Shooter the bad guy. It could be one reason or both reasons. But ultimately, they both resigned from the series abruptly in protest.

Marvel in the 1970's and 1980's, by many accounts, was not an easy place to work for many. I'd love to see a panel of writers and artists who did work for both Marvel and DC, and see their consensus of whether Marvel or DC was the better place to work. Both seem to have had their good and bad moments.