This looks like an upstream problem. Traditional apt mirror format is inherently prone to race conditions. This is fixed in Bug #1430011 “support apt by-hash mirrors” : Bugs : Launchpad by providing a mirror format with hash-based file names, support for which is available since Ubuntu 16.04. But http://apt.postgresql.org/pub/repos/apt/ does not use this new format.
I reported this at https://redmine.postgresql.org/issues/4679.