They are physically the same and there should not be any limitations - R310 and Unleashed R310 are the same product with different firmware. The firmware can be upgraded to Unleashed firmware or to standalone AP (ZoneFlex) firmware and they should work in either case.
Here's a guess: Maybe the Unleashed R310 has a newer/higher firmware build than the rest of your Unleashed network? If that's the case you'll need to manually downgrade the problem unit to match the firmware your other APs are running.
One more guess: What country code are your Unleashed APs set to? US country code will not work with non-US country code, and US country code APs are fixed as US, cannot be changed. This is due to US regulatory requirements.
The "WW-" APs can be changed to US country code, but not the other way around.
Hope it helps!