try setting \setlength\lineskip{0pt} i think you want this anyway and see
what happens.

in addition for testing i would not use dd measures but full pt's which
makes spotting of offsets
far more easy :-)

perhaps this should be taken offline as this isn't really a l3 issue but a
production problem with 2e
i might be able to look into it a bit tonight (though not from this address)


frank

> And curiously in the resulting output the next line of text is offset by
> ~0.4mm.

that is roughly 1pt which is the default for \lineskip