Btw, recently I've also seen the reverse problem. Flight (small feeders inside the US) are avaiable as UA saver awards, but not for redemption with LH miles and not visible on the ANA tool either.
It appears that UA offers some additional inventory on its own flights to its own members, which is not at all unusual. What is unusual is United's decision to disallow many (and sometimes all) awards on partners even if the partners have opened the space up for awards.
Even worse, United's representatives tell people that it is the partner blocking inventory, not UA. This quote from LH confirms that UA is lying to its customers on this issue:
But Martin Rieken, director of corporate communications for the Americas at Lufthansa, said that every Star partner makes the same inventory available on StarNet for any alliance member to use "on a first-come-first-served basis."
Christian Klick, Star's vice president for corporate affairs, agreed with Mr. Rieken's characterization.
"We offer each seat to every frequent-flyer program and have no preference" who uses it, Mr. Rieken said. Both he and Mr. Klick noted that formal redemption agreements, covering issues such as mutual compensation, are negotiated bilaterally between carriers.