I would suspect that you are using a
chapter_title_strip_pattern setting that includes "Chapter *[0-9]+" of some form. It's not in the default, but there is an example in defaults.ini:
Code:
## (Two versions of chapter_title_strip_pattern are shown below. You
## should only have one uncommented.)
## This version will remove the leading number from:
## "1." => ""
## "1. The Beginning" => "The Beginning"
## "1: Start" => "Start"
## "2, Chapter the second" => "Chapter the second"
## etc
## Leaves unchanged "1.1" or "1"
## Note that your ToC entry(ies) may be empty if the
## chapter_title_strip_pattern removes everything and you don't have
## add_chapter_numbers:true.
chapter_title_strip_pattern:^[0-9]+[\.: -]+(?=[^0-9]|$)
## This version remove leading numbers and 'Chapter 1':
## "Chapter 1" => ""
## "1. Chapter 1" => ""
## "1. Chapter 1, Bob's First Clue" => "Bob's First Clue"
## "Chapter 2 - Pirates Place" => "Pirates Place"
## etc
## Note that your ToC entry(ies) may be empty if the
## chapter_title_strip_pattern removes everything and you don't have
## add_chapter_numbers:true.
#chapter_title_strip_pattern:^([0-9]+[\.: -]+)?(Chapter *[0-9]+[\.:, -]*)?