https://www.twitch.tv/ is bookmarked in Opera GX by default. The bookmark is in the bookmarks "Speed Dial" folder so that the bookmark shows up on the start page. This Twitch bookmark has a special (fancy GX) icon for use as a speed dial thumbnail on the start page.
If you're on https://www.twitch.tv/, you should see the heart icon in the address field is already lit up. If you click the heart icon and choose to put the bookmark for the page on the bookmarks bar for example, you'll be moving the bookmark from the speed dial folder to the bookmarks bar folder where that special speed dial thumbnail icon is not supported.
The solution to that is to move the bookmark that's on the bookmarks bar to the bookmarks trash, empty the trash on the opera://bookmarks page, goto https://www.twitch.tv/ (the heart won't be lit up then) and bookmark the page to the bookmarks bar again. Or, if you want to keep the speed dial for Twitch, and still want it on the bookmarks bar, you can use the opera://bookmarks page to right-click the Twitch bookmark in the speed dial folder, choose copy, right-click in the bookmarks bar folder, and choose paste. Or, as you've noticed, you can bookmark another Twitch URL that doesn't match https://www.twitch.tv/.
Now, the same type of thing might happen when importing a https://www.twitch.tv/ bookmark in rare cases, especially if the bookmark is already in a folder when it's imported to the bookmarks bar and then you drag it to the root of the bar. Just know though that it's the same type of issue with https://www.twitch.tv/ already being bookmarked on the start page and having a special icon.
This has been reported to Opera, but no one has fixed it.
