This can happen if you have a sentence where the query terms match many times in the same sentence:
for example if you query on "testing highlighter" but you have
"Testing highlighters is sometimes harder than testing other things."
The issue is that the formatter receives all 3 matches, but in this order:
Testing (first occurrence)
testing (second occurrence)
The formatter expects the matches to be in sorted order by offset (not by term, then offset). This is how the javadocs say they should be.
But there is currently a bug, a stupid side effect of how the ranking is done. Because of this, in this example "highlighters" isnt marked up in bold.