Simulate Rigid Mode Reporting

Good morning community,

I have a general question about how Simulate reports rigid modes when "with rigid mode search" is checked.  I created a simple part with a surface constraint and released 1 DOF (in the Z-direction).  I would expect Simulate to report at least one rigid mode that would show the assembly translating along the Z-direction.  The animation does show this but it doesn't say it is a rigid mode.  Wondering why that is?  Is there a frequency threshold before Simulate reports the mode as rigid?  I released 2 DOF of freedom (in Y and Z) and then it does report rigid modes.  However, when I animate it, there is some rotation of the assembly in the first mode which I would expect only translation.  I attached my assembly file.