Hi Felipe,
It looks like this is a page refresh bug and you can file a support ticket. For now, I reported this in Mendix community Slack in community_feedback channel.
I am also having same issue when using new UI, once I create a story and save. Edit the same story, add label and upon save the screen is not getting refreshed. (This issue only occurs when you create a new story and edit it straightaway to add label and save it). It works as expected for existing stories.