Sorry I don't have a game log here, but in a recent game I Developed a 5-cost card in a game with no 6-cost cards besides gold, and it immediately topdecked a gold (without giving me an option), putting it second from the top. The other card I gained (which was, I believe, a fortress) was topdecked afterwards. It appeared that, since there was only one 6 coin card, I didn't get to choose the order of topdecking.
If this is a bug, it would presumably occur in any scenario with only one X-cost card.
Was it this game: 6349763
The gold was from SirVanders when trashed trigger.
Looks like it worked correctly.
Turn 16 - Rabid
R plays a King's Court.
R plays a Develop.
R trashes a Sir Vander.
R gains a Gold.
R gains a Gold and a Port.
R plays a Develop again.
R trashes a Copper.
R plays a Develop a third time.
R trashes a Copper.
R draws 2 Coppers, a Gold, an Estate and a Port.
Maybe it was.