$ cat ruff_bug.py
'\n'.join([r'line1','line2'])

$ uvx ruff --version
ruff 0.12.8
$ uvx ruff check ruff_bug.py --no-cache --isolated --fix --unsafe-fixes --select FLY002
Installed 1 package in 1ms

error: Fix introduced a syntax error. Reverting all changes.

This indicates a bug in Ruff. If you could open an issue at:

    https://github.com/astral-sh/ruff/issues/new?title=%5BFix%20error%5D

...quoting the contents of `ruff_bug.py`, the rule codes FLY002, along with the `pyproject.toml` settings and executed command, we'd be very appreciative!

ruff_bug.py:1:1: FLY002 Consider f-string instead of string join
  |
1 | '\n'.join([r'line1','line2'])
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ FLY002
  |
  = help: Replace with f-string

Found 1 error.
[*] 1 fixable with the --fix option.