Let's say we want to replace the text within the square bracket hel[__lo__ wo]rld, you won't feel terribly wrong with bolding the newly inserted text, right?  
MS Word seems doing this. LO is bit inconsistent.
hel__l__[__o__ wo]rld  bold
hel[__lo__ wo]rld          normal
hel__l__[__o__ world]  normal