FTR the show runners and the author had a lot of creative differences throughout the show (and especially in the later seasons) [1]

> "It can be... traumatic. Because sometimes their creative vision and your creative vision don't match, and you get the famous creative differences thing — that leads to a lot of conflict," he told Fast Company.

1: https://www.thesun.co.uk/tvandshowbiz/10034023/george-rr-mar...