During the May 11th[1] telecon the paper

   P2286R8: Formatting Ranges

was reviewed.

There were concerns raised regarding the lack of specifications for
determining the boundaries of ill-formed code unit sequences. We decided
it was not a big issue since:
- the method used does not appear to be observable since each code unit
  of the sequence is written to the output anyway.
- it should not matter for self-synchronizing encodings.

I'm working on the implementation of this part of the paper in libc++
and I'm having concerns with example 5 [2]

  string s5 = format("[{:?}]", "\xc3\x28"); // invalid UTF-8
                                            // s5 has value: ["\x{c3}\x{28}"]

\xc3 is the start of a 2-byte UTF-8 code unit sequence
\x28 is not a valid successor byte 
     it is a valid 1-byte UTF-8 sequence for LEFT PARENTHESIS

Based on Chapter 3 of Unicode 14 [3] Constraints on Conversion Processes

  If the converter encounters an ill-formed UTF-8 code unit sequence
  which starts with a valid first byte, but which does not continue with
  valid successor bytes (see Table 3-7), it must not consume the
  successor bytes as part of the ill-formed subsequence whenever those
  successor bytes themselves constitute part of a well-formed UTF-8 code
  unit subsequence.

I would have expected the output to be ["\x{c3}("]. So all code units
are written, but it isn't clear what the exact specification is.