Just make sure all boxed text is at the start of a fresh HTML file.
Graphic image of the text in a box worse than a break, but a start of new file always gives a page break on everything (except stupid apps or apps with a scroll setting), even mobi.
I avoid more than two levels of Header as some ereader system TOC can only do two levels. Instead use a <p of same style. Both of these are easy in Word or LO Writer source (always edit odt in Writer and extra Save As in docx for Calibre or Sigil import) as you can make any user defined heading style be body level instead of an outline level.
If I need more than 2 levels in contents / index I just build that as semi-manually created source inline contents or index page, again easy in Word or LO Writer source