Invalid Unicode in unquoted atom

When I compile hex package io_ansi_plus, I get error “Codepoint failed”. Why? and why are the “Got:” and “Hint:” codepoints identical? It seems in an unquoted atom that Pinyin character ǜ (Latin Small Letter U with diaeresis and grave) is invalid. However Pinyin character ǔ (Latin Small Letter U with caron) is valid. I am using Elixir v1.19.5 when getting this error. In version v1.18, the same source code compiled fine! Here is the error:

mix compile
Compiling 2 files (.ex)

== Compilation error in file lib/io/ansi/plus/ie.ex ==
** (SyntaxError) invalid syntax found on config/persist_colors.exs:2321:13:
error: unexpected token: Codepoint failed identifier tokenization, but a simpler form was found.

  Got:

      "cōng_lǜ" (code points 0x00063 0x0014D 0x0006E 0x00067 0x0005F 0x0006C 0x001DC)

  Hint: You could write the above in a similar way that is accepted by Elixir:

      "cōng_lǜ" (code points 0x00063 0x0014D 0x0006E 0x00067 0x0005F 0x0006C 0x001DC)

  See https://hexdocs.pm/elixir/unicode-syntax.html for more information.
  │

2321 │ :cōng_lǜ_green,
│ ^
│
└─ config/persist_colors.exs:2321:13
(elixir 1.19.5) lib/code.ex:628: Code.validated_eval_string/3
(elixir 1.19.5) lib/config.ex:326: Config.eval!/3
(elixir 1.19.5) lib/config/reader.ex:102: Config.Reader.read!/2
lib/io/ansi/plus/ie.ex:4: anonymous fn/1 in :elixir_compiler_2.MODULE/1
(elixir 1.19.5) lib/enum.ex:961: Enum.“-each/2-lists^foreach/1-0-”/2
lib/io/ansi/plus/ie.ex:4: (module)