When character limit is set (advanced options) the character limit for a comment is decreased by one which each newline.
The same does not apply to issue description/environment (correct limit is used).

Reproduction steps:
1. Set character limit in JIRA to some value (for example 10).
2. Create a project and an issue
3. Open the issue and enter a comment on the issue of maximum allowed length minus one (in this example 9) and a newline. This amounts to 10 characters in total which does not exceed the limit.

Expected:
Comment is created

Actual:
There's an error and comment is not created.

Additional info:
When creating a comment using "edit" issue or with a middle click on "Comment" button (opens up a form in a separate tab) it displays error message about text exceeding the character limit.