Happened to me the other day, I casted Scourge Supplies and the card in the middle (Tour Guide) was not an option to discard, the game only gave me the other 2 options. 
And this “forum” will not let me share any links even preformatted or blockquote buttons… Nice Blizzard 
If scourge supplies draws a “cast when drawn” card, that’s the normal behavior.
If your scourge supplies drew card 1, card 2 which was cast when drawn, card 3 drawn by card 2, card 4, it will only allow you to discard either card 1 or 4. Card 2 no longer exists and card 3 wasn’t drawn by scourge supplies but by the natural draw of a cast when drawn card so it is not an option.
If it’s what happened to you, that’s not a bug