I'm not sure how widespread this is, but for some PLoS papers I've looked at, the sentences shown in search results is only a subset of the total number of possible matching sentences, or in at least one case, no matching sentences are displayed at all (see search results for 'egl-1' keyword and PMID:30677018).
I don't know if this is because of a formatting issue for gene names in PLoS papers (I seem to recall we have some PDF-to-text conversion issues with PLoS papers for AFP, too).
Thx.