Comment 4 for bug 1655338

This is because in your CSS stylesheet you have two rules:

.P_Chapter {
font-weight: bold;
font-style: normal;
font-size: 1.50em;
text-align: center;
margin-top: 0.00em;
margin-bottom: 5.00em;
margin-left: 0.00em;
margin-right: 0.00em;
text-indent: 0.00em;
}

.P_CHAPTER {
font-weight: bold;
font-style: normal;
font-size: 1.50em;
text-transform: uppercase;
text-align: center;
margin-top: 0.00em;
margin-bottom: 5.00em;
margin-left: 0.00em;
margin-right: 0.00em;
text-indent: 0.00em;
}

See the text-transform: uppercase in the second rule. When converting,
calibre matches class names case-insensitively, as that is what the vast
majority of documents use. Therefore, the two rules get combined leading
to what you see. In general you should never have CSS rules with
different cases for the selectors as that will lead to lots of problems
in lots of places.

 status invalid