The code is as follows:

public TextSegment searchText(String searched, PositionInParagraph startPos) {
  ...
   int beginRunPos = 0, candCharPos = 0;
   for (int runPos = startRun; runPos < rArray.length; runPos++) {
            int beginTextPos = 0, beginCharPos = 0, textPos = 0, charPos;
 ....
}

beginTextPos and beginCharPos are in the loop and can be wrongly reset. 

NPOI fixed a similar bug:

https://github.com/nissl-lab/npoi/commit/38cdadd3a67e59e6bede7f77996fbf048c5ffc7e

In NPOI, the fixed code is as follows:

public TextSegment SearchText(String searched, PositionInParagraph startPos){
...
int beginRunPos = 0, beginTextPos = 0, beginCharPos = 0,candCharPos = 0;
    for (int runPos = startRun; runPos < paragraph.GetRList().Count; runPos++){
 int beginTextPos = 0, beginCharPos = 0, textPos = 0, charPos = 0;
...
    }