Nexus will not deliver files from the on-disk cache of a proxy repository if their metadata/artifact max age has expired and the remote is not reachable.
To reproduce this, set up a proxy repository in Nexus 3 and populate it with some artifacts from the remote. Then make the remote unreachable, and invalidate the cache of the proxy.
Try to download the cached artifacts. This will fail with a 502 error.
This is a regression from Nexus 2, and is a serious bug. One of the main purposes of a proxy repository is to protect builds from network issues affecting remote access of repositories.
Manually blocking remote access in the proxy's configuration does not provide a workaround, Nexus attempts remote access anyhow. Retrieval through a group repository also does not work.
I've attached a support zip from the test instance I used to reproduce this issue.