iMovie: Edits from one project affect separate projects

I'm using iMovie 10.4.2 on Mac Studio Apple M1 Max, macOS Sonoma 14.4.1. I created 2 separate libraries, CastA.imovielibrary and CastB.imovielibrary. I created an event called CastA in the CastA.imovielibrary. I created and event called CastB in the CastB.imovielibrary. Into CastA event I imported media file called CastA.mp4. Into CastB event I imported media file called CastB.mp4. I then selected CastA event and created a new project using the CastA.mp4 video. I edited CastA project deleting clips I didn't want in CastA project. I then selected the CastB event from the CastB.imovielibrary. The CastB.mp4 that I imported into the CastB event has the edits that I did in the CastA project!!!! How can this be? FYI - CastA.mp4 and CastB.mp4 is the same ballet BUT with different performers in the cast which is why a need to edit them separately. My granddaughter performs as one character in CastA and a different character in CastB.