Workshop collections needs UI updates.
Presently it's possible to lock your own workshop collection by adding too much to it. In an attempt to create a synced profile for mods I found it's easy to make it impossible to edit your own collection. There's a limit of 1000 mods per collection, going beyond this hides all the content of the collection along with the ability to make changes from the editor. There are many ways to do this but I've found if you add all your subscriptions from a game where you're over this limit Steam will not notify you that you're beyond the 1000 item limited, then allow you to save it and give you a locked collection as a result.

Now there are many solutions Steam Workshop could go about to fix this. Notably removing or raising the 1000 item cap. But to make it functional with the current limit the editor needs to be clear when you're approaching the limit, and still give you a way to split your collection to a new one to stay below the limit. Right now we lose access to any route to fix it within the collection editor, and it should probably be addressed so we stop seeing the "There's no items in this collection" when you can verify the items are there just not displayed.