[FFmpeg-user] Question regarding .gif thumbnailing

[ibwtf]

Hello. I'm using ffmpeg to generate thumbnails for my JavaScript web project. Now I'm having a little problem with .gif thumbnails. Original gif: https://files.catbox.moe/frrsev.gif my software's thumbnail: https://files.catbox.moe/3rtv3o.gif

As you can see, the thumbnail is not perfect and kind of buggy. This only happens with transparent .gifs. The filter i'm using is: [0:v] scale=${thumbSizeFilter},split [a][b]; [a] palettegen=reserve_transparent=on:transparency_color=ffffff [p]; [b][p] paletteuse

Any ideas on how to improve it? Thanks.