Finding text within a presentation that is after a hyperlink is "flaky" --
flakyness ranges from NOT being able to find text which is quite clearly there,
to finding the text but highlighting the *wrong* text.

The problem seems to be that the URL is considered to be part of the source
text.  Specifically, consider the HTML fragment:

  <a href="http://example.com">Hello!</a> world!

Try the above in Impress, and search for "world" -- it won't be found.

Alternatively, try

  <a href="http://example.com">Hello!</a>  Plus lots more text

Search for "Plus" and "lots" will get highlighted instead.

This also impacts Replace as well.