Not sure whats happening at your end.
Seems to be working fine here. Completely transparent at the beginning and transitions on with a wipe from the right.
Perhaps its something weird with the underlying video and not the graphic.
I remember an anomaly last year where things would not key properly if there was a Master Clip LUT on the video. It would key fine if the LUT was applied in the timeline via the Lumetri Effect. Problem only occurred when the LUT was applied at the Master Clip Level. Just for laughs, I tried a Master Clip LUT here and there was no problem, either. So it does not appear to be that, but I just thought I would mention it.
Try background video of a different codec and see if the problem persists.