Could reprod in 28.2 and 29.0.91:

1. Open the attached text file, or save the following in a file and open
   it (hopefully displayed correctly here in your email client...)
--8<---------------cut here---------------start------------->8---
   The issue is not with ’, but the  (nul, insert with C-q C-@).
--8<---------------cut here---------------end--------------->8---

2. M-x set-buffer-file-coding-system utf-8 <RET>

3. A warning appears, attributing the issue to the ’, the quote (in the
   following I have replaced the chars with literal strings

--8<---------------cut here---------------start------------->8---
These default coding systems were tried to encode the following
problematic characters in the buffer ‘encoding.txt’:
  Coding System           Pos  Codepoint  Char
  utf-8-unix               23  #x3FFFE2   \342
                           24  #x3FFF80   \200
                           25  #x3FFF99   \231

However, each of them encountered characters it couldn’t encode:
  utf-8-unix cannot encode these: \342 \200 \231

Click on a character (or switch to this window by ‘C-x o’
and select the characters by RET) to jump to the place it appears,
where ‘C-u C-x =’ will give information about it.

Select one of the safe coding systems listed below,
or cancel the writing with C-g and edit the buffer
   to remove or modify the problematic characters,
or specify any other coding system (and risk losing
   the problematic characters).

  raw-text no-conversion
--8<---------------cut here---------------end--------------->8---

Despite the warning, the correct fix is to remove the nul character.

This can be quite misleading, especially when one wants to fix encoding
issues in big text files.