i know this is a pretty old post but i just figured it out and saw that there's no solutions given yet for this thread! change the bookmark address from https://www.twitch.tv/ to https://www.twitch.tv/?app=desktop it should look like this:

it's honestly so weird that we have to add ?app=desktop after the / for the icon to show but no more blank icon so at least there's a workaround for it! this also works for YouTube as well. (same thing, just add it after the / at the end of the url.)

those were the only two bookmarks i have saved that have an issue with blank logos, if there's any other blank logo bookmarks then this might work for them too!