highlighting non marked words

Hi, 

In my Epub - Adobe DRM book the highligting function doesn't work properly from time to time. Sometimes the marked word och or group of words highlights but many times a word/group of words highlight (-s) instead. It's an adjacent part of the text that is highlighted. 

My (online) bookshop denies any wrong with the book and says Digital Editions malfunctions. 

The attachement shows the marked word "patient" and how a row of letters is highligted in yellow after rightclicking and clicking on "highlight" a few lines above. 

Is my Digital Editions 4.5.12.112 malfunctioning?

Bjoern