I do not see a problem. Slight pivoting as one plays higher to me is normal. Playing in the lower register, I believe, requires a looser, freer vibration of the lower lip and a slight upward pivot of the horn may allow for this.
I am of the opinion that you are doing fine and concern is unwarranted. But this opinion comes from someone who has never had a lesson other than band classes 46 years ago. I decided to pick up the horn again for fun. So we will see what Maestro Spence has to say.