russian category prefix doesn't work after 3.1 update

Still works when i use english prefix.

Already tried re-saving permalink structure (.htaccess is writable) and deactivating plugins.

In fact, wordpress now cuts away the russian part of URL, so when i go to https://example.com/%prefix%/%category%/ i got redirected to https://example.com//%category%/.

Russian tag prefix still works fine, which is kinda surprising.