The calibre conversion feature is designed for reflowable books. It does not support fixed layout formats such as Kindle comics.
My
KFX Input plugin has a command line interface (CLI) that can be used to perform conversion of KPF to EPUB bypassing calibre's conversion pipeline. That will retain the fixed layout formatting of a comic. And the
KFX Output plugin command line interface can convert KPF comics to KFX.
If you are instead starting from a KF8 (azw3) file you can use the
KindleUnpack plugin to extract a fixed layout EPUB without conversion.