Light Flare transparency trouble

I know its a bit late for you, but since I came up with the same problem today and your question was the only one I could find with the same kind of problem I ll share how I solved this.

Basically just forget about alpha at all, I made a Sun flare with a single texture and 2 layers on Gimp, 1 layer is a plain black opaque background and the other one contained the actual Sun, exported as PNG but with the alpha channel completely filled up, didn't configure anything in unity, only the default export with the default settings ll do, this solved the problem for me at least.

Cheers and thx for the opportunity.