I looked back and something is clearly wrong with my coverage reporting setup, sorry :(

When I move the test introduced in issue 17909, 'test_bytes_decode' from the module Lib/test/test_json/test_unicode.py to Lib/test/test_json/test_decode.py that particular test is properly traced and I see that the function is mostly covered, so I need to figure out what is going wrong in my setup.

The test already present is more comprehensive as it includes BOM, so I believe what I wrote is generally not helpful.

I did notice that the existing test does not cover the edge-case of a 2-byte utf-16 sequence that can be generated for a valid JSON object represented by a single codepoint, for example '5' or '7'. I created a new patch to augment the existing test to cover this special case rather than adding a test.