Happens for most if not all particle items. Issue is much more prominent on thumbnails that include sparkles like periastrons or seasonal fairies (e.g. Fall Fairy, Spring Fairy).
Desired appearance:
Actual appearance:
On top of this, there are issues surrounding particles in general - many particles won’t even render unless if it’s in front of an opaque surface.
Particles also lack color when not in front of surfaces like these. You can see in my other post here - Amethyst Periastron Kappa’s particles (purple) only have color when they overlap my body.