I made a video with alpha transparency...
Wow!! How did you do that?
There are surprisingly few video file formats that can contain alpha channel.
Love2d supports only OGV video format https://love2d.org/wiki/love.graphics.newVideo
Just googled it for a bit and couldn't find something about alpha channel inside Ogg Theora.
Are you sure your file contains alpha channel? Can you share it here (not a .gif but .ogv)? Can you see this transparence in some player software?
A few month ago I was looking for some way to store animation with alpha-transparency and found only apng format for me. A good question about supporting it in Love2d
And if the is no such options, then I suggest 2 separate videos: one with color + one with alpha along with simple shader will be simplest solution for your task.