[odamizu]

Sorry for the incomplete info. Per your link above:

1. Source epub attached.
   
   
2. Calibre conversion log:
   Spoiler:

   Convert book 1 of 1 (Header Test2)
   Resolved conversion options
   calibre version: 2.42.0
   {'asciiize': False,
   'author_sort': None,
   'authors': None,
   'base_font_size': 0.0,
   'book_producer': None,
   'change_justification': u'left',
   'chapter': u'/',
   'chapter_mark': u'none',
   'comments': None,
   'cover': u'/var/folders/40/j4qb4w1j68b_y8wpsl9zgtvc0000gn/T/calibre_2.42.0_tmp_zeeLO2/tmPhVL.jpeg',
   'debug_pipeline': None,
   'dehyphenate': True,
   'delete_blank_paragraphs': True,
   'disable_font_rescaling': True,
   'dont_compress': False,
   'duplicate_links_in_toc': False,
   'embed_all_fonts': False,
   'embed_font_family': None,
   'enable_heuristics': False,
   'expand_css': True,
   'extra_css': None,
   'extract_to': None,
   'filter_css': u'',
   'fix_indents': True,
   'font_size_mapping': None,
   'format_scene_breaks': True,
   'html_unwrap_factor': 0.4,
   'input_encoding': None,
   'input_profile': <calibre.customize.profiles.InputProfile object at 0x10babe350>,
   'insert_blank_line': False,
   'insert_blank_line_size': 1.0,
   'insert_metadata': False,
   'isbn': None,
   'italicize_common_cases': True,
   'keep_ligatures': False,
   'language': None,
   'level1_toc': u'//h:h1',
   'level2_toc': u'//h:h2',
   'level3_toc': None,
   'line_height': 0.0,
   'linearize_tables': False,
   'margin_bottom': -1.0,
   'margin_left': -1.0,
   'margin_right': -1.0,
   'margin_top': -1.0,
   'markup_chapter_headings': True,
   'max_toc_links': 50,
   'minimum_line_height': 0.0,
   'mobi_file_type': u'both',
   'mobi_ignore_margins': False,
   'mobi_keep_original_images': False,
   'mobi_toc_at_start': False,
   'no_chapters_in_toc': False,
   'no_inline_navbars': False,
   'no_inline_toc': False,
   'output_profile': <calibre.customize.profiles.TabletOutput object at 0x10babef50>,
   'page_breaks_before': u'/',
   'personal_doc': u'[EBOK]',
   'prefer_author_sort': False,
   'prefer_metadata_cover': False,
   'pretty_print': False,
   'pubdate': None,
   'publisher': None,
   'rating': None,
   'read_metadata_from_opf': u'/var/folders/40/j4qb4w1j68b_y8wpsl9zgtvc0000gn/T/calibre_2.42.0_tmp_zeeLO2/lELScf.opf',
   'remove_fake_margins': False,
   'remove_first_image': False,
   'remove_paragraph_spacing': False,
   'remove_paragraph_spacing_indent_size': 1.0,
   'renumber_headings': True,
   'replace_scene_breaks': u'',
   'search_replace': '[]',
   'series': None,
   'series_index': None,
   'share_not_sync': False,
   'smarten_punctuation': False,
   'sr1_replace': None,
   'sr1_search': None,
   'sr2_replace': None,
   'sr2_search': None,
   'sr3_replace': None,
   'sr3_search': None,
   'start_reading_at': None,
   'subset_embedded_fonts': False,
   'tags': None,
   'timestamp': None,
   'title': None,
   'title_sort': None,
   'toc_filter': None,
   'toc_threshold': 6,
   'toc_title': None,
   'unsmarten_punctuation': False,
   'unwrap_lines': True,
   'use_auto_toc': False,
   'verbose': 2}
   InputFormatPlugin: EPUB Input running
   on /var/folders/40/j4qb4w1j68b_y8wpsl9zgtvc0000gn/T/calibre_2.42.0_tmp_zeeLO2/7l9cuH.epub
   Parsing all content...
   Parsing OEBPS/Text/Header-Test2_0001.html ...
   Parsing OEBPS/Styles/headertest.css ...
   Parsing OEBPS/Text/Header-Test2.html ...
   Reading TOC from NCX...
   Merging user specified metadata...
   Detecting structure...
   Flattening CSS and remapping font sizes...
   Source base font size is 12.00000pt
   Cleaning up manifest...
   Trimming unused files from manifest...
   Creating MOBI Output...
   Serializing resources...
   Splitting markup on page breaks and flow limits, if any...
   Creating KF8 output
   Generating in-line ToC
   Generating KF8 markup...
   Compressing markup...
   Creating indices...
   Creating MOBI 6 output
   Generating in-line TOC...
   Applying case-transforming CSS...
   Parsing manglecase.css ...
   Parsing tocstyle.css ...
   Rasterizing SVG images...
   Converting XHTML to Mobipocket markup...
   Serializing markup content...
   Compressing markup content...
   Generating MOBI index for a book
   MOBI output written to /var/folders/40/j4qb4w1j68b_y8wpsl9zgtvc0000gn/T/calibre_2.42.0_tmp_zeeLO2/mUHRKD.mobi
   
3. Output dual-MOBI attached. (I have also tried this with straight mobi with the same problem.)
   
4. See first post for description of problem.
   
5. Conversion options: I'm not sure what the original defaults were, but I think most options are reflected in conversion log above. If there's anything in particular you want me to check, please let me know.

Thank you!