The Paste-Special command (Edit > Paste-Special, or Ctrl-Shft-V) should let the user paste Formatted text [RTF] that retains the font, size, and color of the original text. (This is very useful e.g. when you need to insert frequent computer-style commands in typewriter font.) Instead, the formatted Paste loses all original information. It acts no different from the Unformatted option.
The workaround in Impress is apparently to specify EditEngineFormat. But this menu option is never explained, or even mentioned, in Impress Guide 4.0. It is not even present in Writer. The same capability worked in PowerPt, so there seems to be no compelling reason that LibreOffice should not support it.