It turns out that 'online' mode is kinda broken at the moment in the library - that's my fault for introducing it without ongoing testing.  In particular, the 'passthrough' flag -- by-design in 'responses' -- functions on in individual HTTP requests.  So, if an online request is redirected by the server, then -- without further configuration -- responses won't match the subsequent request to the redirected URL.

Although some workarounds are possible, I think this feature needs a redesign.  It would be brilliant to support and improve near-current-data testing against various sites without sending them repeat/spammy request traffic.  'responses' has introduced a beta recording feature, and also VCR.py is available.