I don't agree it's a bug. In your case at least it's a coding issue. Try:
Code:
With Selection.Range.Find
.MatchCase = False
.MatchWildcards = False
.MatchSoundsLike = False
.MatchAllWordForms = False
.Text = "OLD"
.Replacement.Text = "NEWTHING"
.Execute Replace:=wdReplaceAll
.Text = "REPLACEME"
.Replacement.Text = "THEREPLACEMENT"
.Execute Replace:=wdReplaceAll
End With