It's the international holdings reports (it's monthly, for May) - bring it up on your Bloomberg and read it

Russia sold about 50 yards in April and apparently got rid of pretty much everything in May, thus it dropped from the foreign holders report completely.
That the overseas flows don't seem to explain the direction of the bond market (at least not this time around). This said, overall purchases of US assets (different report) seem to be down a fair bit (230 yards in April down to 70 in May). Not trying to argue any view in particular, just throwing in some data points.