Transparency Issues in library previews (since like three months)

I opened the Libraries hoping that the problem was finally solved, but it's sadly not. I'm attaching two screenshots with the vector I uploaded a few moments ago.

This also happens with old files that I already have in my library for more than two years... Eveything is broken, it's a hit and miss...

Examples:

Left: Vectors, no background. Transparency grid to show it (Shift+Cmd+D).
Right: Library panel with the same vectors with that white background that I never added.