I'm having exactly the same issue. I have followed the same steps that you. I have also used shutterstock to get the video in H2.65 HEVC codec and .mp4 format for Safari enabling alpha channel. It works perfetly on Mac but not on iphone. In my case I see a white background behind my video with alpha channel where it should just be transparent.
It would be of inmense appreciation if someone could help with this.
nestornavas
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