I recently tried to add my friend to a Team Create, and when I enabled Team Create, there was no bar for me to type his username. I tried this on multiple games, and even games I had used Team Create on before, and the bar still did not show up.
Sometimes, when I would enable Team Create on a new place, the game would fail to load entirely not allowing me to access it in Studio at all afterward.
As I do not use Team Create that often, I cannot provide exact information on when it started occurring.
Here is an example of what the UI should look like:
This is what it actually looks like: