last week i have formatted my wiiu and didnt appears the iconsI'm not sure if the default icons r located in the mlc or not. they may be. formatting should correct that if that were the case though, so they may be located elsewhere. with formatting, they would've been recreated by some flash file or simply untouched. it's difficult to say really.
downgrading can't be done afaik. what happens with the 3ds is that it detects missing tiles and redownloads them. there's no downgrading involved. that's y I suggested it. I've never heard of this problem either, so I'm just guessing at this point.
before i created this topic i seen two similars
https://gbatemp.net/threads/help-lost-some-system-menu-icons.459469/
https://gbatemp.net/threads/disc-and-wii-icon.451387/
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so i format my wiiu again?@godreborn is correct. I'd backup all your save games that you care about and reformat your Wii u. Then use my guide (in my sig and in the stickies) to setup the latest hacking methods.