It won't help you, but JFYI, it works fine for me, so it does not look like a general bug.
Do you get any error when adding the manual activity? Try loading the page https://connect.garmin.com/modern/activity/manual and if it comes out blank, or with an error message, force-reload the page (Shif-F5). Also try disabling ad blockers, and all other browser plugins.
And if you do not manage to make it working, simply record a dummy 5s activity of any type on your watch, sync, and then edit it to your liking in Garmin Connect.
