There is a bug with links to apps under the ‘apps’ section of the shopify admin nav bar.
If you’re on a screen different to the app, i.e. home, then click on the app, it loads correctly. i.e. https://admin.shopify.com/store/xxx/apps/appname?querystring
But if you’re already navigated to that app, then click the link again, it incorrectly redirects you to the app’s domain instead, i.e. https://appdomain.com?querystring
we also observed a similar issue with app-bridge modals having a similar issue a couple of days ago where it would append the modal src to the end of the full app url, i.e if the app landing url was https://appdomain.com/landing.php and the modal src was /a/b/c.php it would incorrectly load the modal with https://appdomain.com/landing.php/a/b/c.php rather than https://appdomain.com/a/b/c.php however this seems to no longer be an issue.