Hi Casualfanboy, thanks for trying Flipnot!
I have to say I'm stumped too... I've never seen image compression completely remove a color. When an image gets resized, on-off patterns like those in manga or pixel art like yours can cause weird effects (see also "ringing"). It looks like the image resizer on your phone (is it an iphone?) loses that color.
I tried looking at it on Discord on my computer and phone (Windows and Android) and it looks fine.