That does seem unexpected. Is that in the Puppet panel, Scene panel, or both?
The torso layer is appearing below the arm layer in the Puppet panel?
If you'd like to share a link to the artwork file (via Dropbox, Google Drive, etc.), via direct message or here, I can investigate what might be going on.
If you're still encountering the issue, could you send a shared link to the artwork file so that we can diagnose the issue? Thanks.
Sorry for the delay. It seems I was able to resolve it. Turns out I had accidentally cut my upper arm into more than on part. A quick pathfinder merge solved the issue. Still a little odd but I'm guessing it was because of the slight movement in the piece. Thanks for responding.
The seam is due to the adjacent edges of paths being near pixel boundaries. If you switch AI to use Pixel Preview mode, you should be able to see fractional alpha edges. They don't sum together, they screen (e.g., 50% over 50% is 75%, not 100%). Creating some overlap or merging as you did is the solution here.
BTW, After Effects has a special Alpha Add blending mode for this, as described here: https://www.provideocoalition.com/after-effects-hidden-gems-weekly-alpha-add-mode/