Pypi proxying is broken between 2 NXRM instances

We have several nexuses. The two in question are set up such that:

• `A` nexus has a pypi group `pypi`, with several constituent repositories
• `B` nexus has a pypi proxy repository pypi-upstream, configured with an upstream url of `https://A.example.com/repository/pypi/`

We're seeing it fail to proxy artifacts correctly.

If we view `https://B.example.com/repository/pypi-upstream/simple/python-dateutil/` it shows no links. Invalidating the cache or rebuilding the index doesn't help.

A tcpdump on nexus A shows that it is indeed sending a request, however the request it sends has an If-Modified-Since header, which consequently returns no results.

Nexus B accepts this, fills it's cache with a blank result (which is nonsense), and proxying is thus failing.

(Querying nexus A using curl presents many links for `https://A.example.com/repository/pypi/simple/python-dateutil/`, the packages are definitely there)