No, only for the current conversation. If you want to save it in the save system, you can do that separately.
The exception is if you have used the SetPortrait() sequencer command to specify a portrait image that's not in the actor's portraits list.
SetPortrait(Bob, pic=2): Will make Bob use portrait 2 in its portraits list for the duration of the conversation.
SetPortrait(Bob, default): Will make Bob use its default portrait 1 for the duration of the conversation.
SetPortrait(Bob, SpecialImageXYZ): Will load a sprite named "SpecialImageXYZ" from a Resources folder, asset bundle, or Addressables and use it until changed. Will be included in the save system. This is because this is the only reference to the image since it's not in the portraits list.