Sometimes, when trying to test the events in the Google Tag Manager preview, the events show “Successfully fired;” however, they are “Unknown Tag Type,” and the data values are blank.

Preview periodically calls the server to render whatever has happened in your particular preview session; as such, there are moments when only part of the information related to any given request/tag has loaded. 

You should just wait a bit or send some more events, and eventually, you’ll see the actual status/info of tags. Often, it may be necessary to completely close and reopen the preview mode to make this problem go away.