Nebula poofs with transparency are too bright

This requires a particular set of circumstances...

Scroll of Atankharzim uses custom nebula poofs. These worked fine in 3.7.4, but as of 3.8.0, they are much too bright:
https://cdn.discordapp.com/attachments/223511363807346691/662690627128918049/unknown.png

However, removing the post-processing command-line flag allows them to display at their proper brightness.

I noticed that the Scroll poofs appear to have a transparent background, while the MediaVP poofs (which display fine) have a black background. However, Scroll and MVP poofs alike have alpha channels.

Removing the alpha channel, or merging the poof with a back background, does not work for Scroll because this happens:

I used DXT5 compression and "generate mipmaps" for modifying the Scroll poof.