Property tracks rely in the default display name for naming in the UI. String tracks have their own override for the default display name getter (UMovieSceneStringTrack::GetDefaultDisplayName) which always returns "Strings". This isn't an issue normally, but when adding the track through scripting the default name is used.
The added track is named "Strings" instead of the variable's actual name
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-103567 in the post.