Issue with using additive animation clips imported from one .FBX

I'm running into an issue with getting additive animations to work when importing from an .FBX. My .FBX animation should be played in parts of 150 frames (so, play till 150, pause, play till 300, etc..). What I did to achieve this is divide my .FBX animation into clips (see below).

It's a progressive animation that draws a horizontal line through the screen from left to right), where each clip accounts for 1/5th of that line, so in the last clip it reaches the far right of the screen.

The issue I'm having is that the moment I transition from the 1st clip to the 2nd clip, the horizontal line from clip 1 disappears, and when I transition from the 2nd clip to the 3d, the line from clip 2 disappears, etc etc. In order to base clip 2 off of clip 1, I attempted to set the 'additive reference pose' of the second clip to the last frame of the 1st clip, but this does not solve my problem.

Would greatly appreciate any help or pointers on what I'm doing wrong. Thanks.