Unwanted vertical shift when using overlay on itemize/enumerate

Question

The following code generate a vertical shift on the item 1. on the next slides. I don't know why and after few try, it's still moving when I go to next slides. Do you know why and how should I do to correct this ? I have the same problem for an other slide when the enumerate content appear on it.

\documentclass[hyperref={pdfpagemode=FullScreen},xcolor=table,t]{beamer}

\begin{document}
\begin{frame}
\frametitle{Frame Title}
\begin{itemize}
    \item<+-> first item
    \begin{enumerate}
        \item<+-> item 1.
        \only<+>{
        \begin{enumerate}[a.]
            \item item a.
            \item item b.
            \item item c.
        \end{enumerate}
        }
        \item<+-> item 2.
    \end{enumerate}
\end{itemize}
\end{frame}
\end{document}

Addition by @samcarter

The problem can be simplified a bit: a nested itemization influences the top alignment of the top-level itemization.

MWE:

\documentclass[t]{beamer}

\begin{document}

\begin{frame}
\begin{itemize}
    \item item 1.
\end{itemize}
\end{frame}

\begin{frame}
\begin{itemize}
    \item item 1.
    \begin{itemize}
        \item item a.
    \end{itemize}
\end{itemize}
\end{frame}

\end{document}

Answer 1

Workaround: start the nested enumeration in a new paragraph by leaving an empty line in front of it:

\documentclass[hyperref={pdfpagemode=FullScreen},xcolor=table,t]{beamer}

\begin{document}
\begin{frame}
\frametitle{Frame Title}
\begin{itemize}
    \item<+-> first item
    \begin{enumerate}
        \item<+-> item 1.

        \only<+>{
        \begin{enumerate}[a.]
            \item item a.
            \item item b.
            \item item c.
        \end{enumerate}
        }
        \item<+-> item 2.
    \end{enumerate}
\end{itemize}
\end{frame}
\end{document}