herolulu Okay, so see the videos below. The copy-and-paste should have been done in the following way (To make the problem easier to understand, I reproduced the problem with an animation leaving only the path position animation.):
To fix this, delete the middle key and double the value of the last key to the original value:
The reason for doubling the value of the last key is that if the original value is left unchanged, it would take twice as long to reach the specified path position. (This is the state shown at 0:02-0:06 in the video above.) To make it move at the same speed, we need to double the value.
For a closed path, changing the path position from 0 to 100 means that the constrained bones will move along the path. So if we think in terms of the constrained bones' coordinates, their coordinates at path position 0 and at path position 100 will be the same. However, the value that the path position keys have is different, and this is what we need to be aware of when copying and pasting the key.
Next, let's look at how to set the path position to 100 at frame 59.99 and back to 0 at frame 60. To set the key at frame 59.99, either hold the SHIFT key and move the timeline bar to set the key there, or use the Current field of the Dopesheet view to move the timeline bar to frame 59.99. In the following video, I used the former approach. I then selected the key at frame 59.99 and switched the curve to stepped.:
If you copy and paste this animation to repeat it, there is no need to modify the path position values after pasting:
For your information, a frame like 59.99 is called a fractional frame. This is explained on the Keys page of the User Guide: https://zh.esotericsoftware.com/spine-keys#%E5%B8%A7%E5%AF%B9%E9%BD%90
