So i did some more testing, and Shella ex up attack only behaves the way i described when Shella is on the right side of the screen. So that problably means that it's not a problem with the hitbox itself, but with the way the game handles the characters when they are on the right side or something like that. Or Weenter just messed up something by accident lol, but idk.
Made an emergency update that fixes the Shella hitbox bug. it was a bug indeed. When spawning the thunder hitbox, it was accidentally flipped for 1 frame, but that is fixed now.
Mad's dash attack also becomes harder to combo the more you use it.
Also, good observations :) and thanks for the bug reports.