Hey there! It's great that you're looking to solve this. The "Open in app" option for Pixiv might have changed due to updates. You can try clearing your browser's cache and cookies first. If that doesn't work, you can also check if there's an update for the Pixiv app itself. If it's still not working, you could use the share option and see if the app appears in the list. Also, remember that sometimes these options can change based on the app's compatibility with different browsers.