Okay, I can appreciate avoiding extra trouble for others, and it’s certainly easier for me to add the workaround translation for just this one case. I’ve now done that (with your suggestion, which was probably as good as this can be).

%{month_name} %{year} would work better for Finnish, as that would allow the month name to be inflected apart from the year (which is how the current accepted translation of this string was intended to work). OTOH, the Finnish way a ”month name + year number” combination in inflected in most, if not all (relevant) cases is like that: add a suffix to the month name, leave the year number as-is. Without workarounds, %{month_year} generally only works for the nominative case.