This went under the radar for a few days, but MediaWiki 1.20wmf11
brought some major changes to plural handling, thanks to the new CLDR
plural parser. Careful release notes reader could have spotted it here:
Niklas has just added some (hopefully temporary) overrides to restore
the old behaviour and fix https://bugzilla.wikimedia.org/40251
should report on the bug if your language has the same problem.
lists the differences between CLDR and MediaWiki rules and
has some more discussion.
P.s.: 1.20wmf11 has been enabled everywhere on the 12th and the switched
on and off a few times on en.wiki but it seems it will stay now despite
some other bugs, check [[Special:Version]].