That is interesting. I can reproduce it.
You know, to get an identity transformation without re-instantiation, you can also use the IDENTITY constant (Geom::Transformation::IDENTITY). But as it seems the use case checking whether a given transformation is identity is not provided by the API. I wonder whether it could really be a bug or rather by design (a flag in the transformation object). After all, numeric discrepancies in transformations can have much more impact than in lengths. Comparison of the floats of the transformation matrix could maybe not guarantee that the transformation really behaves like an identity.