You should not use sys.getfilesystemencoding() nor locale.getpreferredencofing() to check if the C locale has been coerced, but read the LC_CTYPE locale instead:

# C locale coerced
$ LC_CTYPE=C python3 -c "import locale; print(locale.setlocale(locale.LC_CTYPE, ''))"
C.UTF-8

# C locale not coerced: remains "C"
$ LC_CTYPE=C PYTHONCOERCECLOCALE=0 python3 -c "import locale; print(locale.setlocale(locale.LC_CTYPE, ''))"
C

--

$ LC_CTYPE=C PYTHONCOERCECLOCALE=0 python3 -c "import sys; print(sys.getfilesystemencoding())"   
utf-8

In this example, the UTF-8 encoding is used because of the UTF-8 Mode:

$ LC_CTYPE=C PYTHONCOERCECLOCALE=0 python3 -X utf8=1 -c "import sys; print(sys.getfilesystemencoding(), sys.flags.utf8_mode)"
utf-8 1

$ LC_CTYPE=C PYTHONCOERCECLOCALE=0 python3 -X utf8=0 -c "import sys; print(sys.getfilesystemencoding(), sys.flags.utf8_mode)"
ascii 0

The relationship between the PEP 538 and the PEP 540 is non obvious:
https://www.python.org/dev/peps/pep-0540/#relationship-with-the-locale-coercion-pep-538

I guess that you would like: PYTHONUTF8=0 env var or -X utf8=0 command line option.

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I don't think that this issue is a bug, and so I suggest to close it as "not a bug".