You refer to the "original text"... I presume this is a copy/paste operation?
If the copy is from a pdf document, that has the font embedded, and you do not have the font installed, then I imagine something like this would happen...
If, however, this is not where you are copying from, but rather you are copying from something else on your PC, this suggests that you do have the font installed...
Which means it's probably am issue with Word.
Have you tried searching on Microsoft to see if it's a known issue?