As MarcelG said, the place I originally put the new code caused the html to be displayed (not converted back to forum code) when editing a previous post that had the new tag in it. I move the code to line number 393 in in_func_comon.asp in the extratags area and it now works correctly. When you go to edit a post that the new tag was used in, it correctly changes it back to forum code when retrieving it from the database.
fString = doCode(fString, "Reveal hidden content", "", "<font color=""" & CColor & """>", "</font id=""" & CColor & """>")
fString = doCode(fString, "[t]", "[/t]", "<dl><dt><dd>", "</dd></dt></dl>")