I am having the exact same problem, and I’m sorry to see no one has answered you. Did you find a solution?
I see another user in a different thread had the same problem, but his situation was complicated by Time Remapping, so the answers there were not helpful (and his problem went unsolved). Not so here. This is definitely the same problem.
I am trying to do a fairly simple animation of a screen capture of a news article.
Motion 1: scroll down (only vertical Position value changes),
Motion 2: zoom in on right-hand column while continuing to scroll down (horizontal, vertical Position values change; scale changes)
Motion 3: continue scrolling down (only vertical Position value changes).
Problem emerges before I get to the third keyframe. The graphic is meandering like a planet between the first two keyframes, in which ONLY the VERTICAL POSITION value changes. The horizontal values of the two keyframes are the same (360), but the values in between rise and fall, reaching a height of 375.1 in the middle!

First keyframe.

Second keyframe.

In between the two.
I have tried deleting and redoing keyframes, and re-importing the graphic; it’s the same thing each time. It’s as if PP was adding invisible keyframes between the keyframes I create, and throwing in arbitrary values. It’s maddening. Any ideas?