From a German user perspective, ICU and your fancy library are incorrect, actually. Mass is not a different casing of Maß, they are different characters. Google likely changed this because it didn't do what users wanted.
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However, it is likely that it has never been pronounced "sz", but always "ss" and the habit of writing "sz" for the double consonant may have had the same reason as the writing of "ck" instead of the double "kk".