While cryptic, and not exactly correct—see the UPDATE appended to my post before I received this reply, in short: there is no launching of a pager—this did get me to insert some print statements and compare the output in the two scenarios. The heart of the problem was indeed the lack of TTY, and a cryptic line from 8 years ago that I no longer grok the intent of:
in reply to Re: Otherwise successful test fails under test_harness (-t)
in thread Otherwise successful test fails under test_harness
# This allows us to have multiple pseudo-STDOUT
return 0 unless -t STDOUT;
In Bob We Trust, All Others Bring Data.