Hi, I'm having the same issue. It seems to be since the reintroduction of the simplified curve editor in V4.1 (which I'm very thankful for!) that it's become an issue.
I've found the issue happens when you make a keyframe and modify the image's rotation or position. If you then add a bezier curve to the movement and modify the top right bezier handle, and then set a new rotate / translate keyframe (for example, 20 frames after), Spine automatically interprets a curve between these 2 keyframes, even though they're identical.
As IdaliaR mentioned, this can be solved by making the end frame of the custom bezier movement a linear bezier, but when you have a 40 second long animation with lots of these move and hold segments, this can get very tiresome indeed.