Did You set "timedMetaDataSelectionKey" field of video node?
For obtaining metadata You have to set timed metadata selection keys, as follows:
SGVideoNode.timedMetaDataSelectionKeys = ["*"]
Setting this array ["*"] should select all timed metadata.
For more reading visit this documentation site and search for "timedMetaDataSelectionKeys ", "timedMetaData " fields.
Although, i think that timed metadata which does not contain start time are not populated by SG video node. But this is my assumption for now, i need to take a deeper look at timedMetaData in roku and documentation.