Hi Andreas,
Was the second “Wait until” tile what you added during editing?
From my test, it seems that even if we can add new tiles to live journey, but contacts are always under processing instead of getting processed.
My journey is a simple email journey, and I added a phone call tile in editing status, however, the tile only shows "processing...".

According to description of "Live, editable" status from documentation:
The journey is currently live, but you have chosen to edit it locally. The live journey will continue to process contacts and take other actions while you work with the local version.
In such status, it allows us to edit fields, while operation like adding tiles is not mentioned.
https://docs.microsoft.com/en-us/dynamics365/marketing/go-live#customer-journey-go-live-operations-and-status

In a word, although tiles are able to be added to journey under editable status, contacts won't be processed by the new tile.
Regards,
Clofly