IB is known to have occasional issues with its Award Availability across different systems and the U Class showing-up in Regular Availability searches is something quite recent, so we don't yet have any authoritative data on this particular scenario...
Is there a pattern to the issues (i.e. does it tend to show availability when there is none, or vice versa)? For example, I just tried to call AA to book said flight and they were unable to find me a seat -- is it more likely that the agent wasn't looking at the system correctly, or that Qantas is returning availability when there is none? I'm guessing the latter, but I typically fly AA metal so I'm unfamiliar with the OW partner process.