`savefig` adds transparency to my picture?

Hi, I have a curious phenomenon here which seemingly appeared from nowhere. I am doing a simple plot (using Plots) and saving it to a file ; but the saved version seems to be transparent.

The MWE is

using Plots
heatmap(rand(8,8))
p=plot!(aspect_ratio=:equal, colorbar=false, axis=([], false))
savefig(p, "test.png")

and here is what I get in my editor : left is my notebook showing the picture, right is the saved picture.

This issue occurs while plotting with default gr backend, but not with pythonplot.

For similar result, use keyword argument border=:none, which works fine with gr():

p = heatmap(rand(8,8))
plot!(aspect_ratio=:equal, colorbar=false, border=:none)