That string was changed back in 2021. It uses %{count} instead of %{min} and has been converted into a pluralized string (one, other).
You can fix your customized string by going to Customize → Text:
- Search for “js.composer.error.post_length”
- click “edit”
- click “revert changes” and confirm the question with “yes”
- click “back to search”
- Search for “js.composer.error.post_length”
- You will see 2 results, customize both strings if you wish