When using OFFSET constraints on an output pad that has both the 3-state flip-flop in a particular IOB and the OUT flip-flop in the same IOB, you must apply different OFFSET constraints to the different flip-flops.
The normal method would be to create one TIMEGRP for the 3-state FFs and another for the OUT-FFs, and then to apply different OFFSET constraints to the different groups. However, when both the 3-state FF and the OUR-FF go into the IOB, one of the OFFSET constraints is ignored. ("0 items analyzed"). However, if one of those flip-flops is moved into a CLB, then both of the OFFSET constraints are analyzed properly.
To work around this problem, create a detailed timing report where it is possible to find the paths through the flip-flops situated in the IOBs, and the calculations can be performed manually.
Another work-around is to run a detailed report with just the 3-state FF-to-pad path, then run another detailed report with just the OUT-FF-to-pad path.