According to this document: https://docs.microsoft.com/en-us/azure/cognitive-services/bing-custom-search/hit-highlighting, highlighting works on title, snippet and links.

When "Highlight words" was turned on in the Hosted UI, it highlighted the words only in snippet and the link but not in the title.

When used in the web API call with textDecorations=true and textFormat is HTML, it highlighted the snippet as:

fixed rate <b>savings</b> bonds rather than formatting HTML. Also we think <b> should be replaced by <mark> tags or <em>

It will also me more useful if we could turn on snippet, title individually so that we can customise to highlight text in title or snippet or both.