It isn't a bug. You are explicitly ignoring the first instance found.
Code:
Sub ClearSpecificHighlight()
Dim oRngStory As Range
For Each oRngStory In ActiveDocument.StoryRanges
Do
With oRngStory.Find
.Highlight = True
Do While .Execute
If oRngStory.HighlightColorIndex = wdTurquoise Then
oRngStory.HighlightColorIndex = wdNoHighlight
oRngStory.Collapse 0
End If
Loop
End With
'Get next linked story (if any)
Set oRngStory = oRngStory.NextStoryRange
Loop Until oRngStory Is Nothing
Next
lbl_exit:
Exit Sub
End Sub