I have noticed that our custom extension is not available in the installed extensions list. It is a cron job extension so when I check the https://manage.auth0.com/#/cron link I can see that extension is actually still available.
For custom extensions, it is required to have the endpoint GET \meta which will serve the webtask.json. Please check that your custom extension has this endpoint.
const metadata = require('./webtask.json');
// This endpoint would be called by webtask-gallery when the extension is installed as custom-extension
app.get('/meta', (req, res) => {
res.status(200).send(metadata);
});