I think that I've understood what happened. It appears that the versions (revisions) of this transition are different (in the project and in the global storage).
You previously applied this transition to slides. Then you modified the transition in the global storage being in another project (changed something). Usually, when a project uses a transition, it syncs automatically, but this didn't happen in your case because the transition wasn't used but hadn't yet been deleted from the project. We'll try to fix this.
Simple solution:
1. Temporarily apply this transition to one of the slides.
2. Click OK and close the project.
3. Open the project (PTE).
4. Untick the transition. Click OK and close the project (PTE).
Now unused resources will be deleted automatically from the project folder.