The portrait name displays "Player" for a split-second before updating and displaying the name of the actor who is supposed to be speaking. This only happens the first time a conversation is started in a scene. This does not happen if I used the dialogue system trigger but I need to be able to start the conversation through code.
A common reason for this is if the subtitle panel's Visibility is set to Always From Start and if there's a mismatch of actors in the conversation.
Check which actors are assigned as the conversation's actor and conversant. To do this, inspect the conversation in the Dialogue Actor and select Menu > Conversation Properties. You'll typically set the Actor dropdown to the Player actor and the Conversant dropdown to an NPC, but you can set both to NPCs if it's an NPC-NPC conversation.