动画帧中的文本移动

Question

我不确定为什么会看到这种行为，但是当我编译以下代码时，在第二帧上，第一行向下移动。我本来\begin{frame}[t]以为这不应该发生，但事实确实如此。我的幻灯片更复杂，有 tikz 图形，但这个 MWE 也展示了我的问题。任何建议都将不胜感激。

\documentclass[table,dvipsnames,aspectratio=149,11.5pt]{beamer} 
\usepackage[draft]{pgf}

\begin{document}
\begin{frame}[t]
    \frametitle{Two Examples}
    
\begin{columns}
    \begin{column}{0.6\textwidth}
        \begin{itemize}
            \item GraphPartition 
            \only<1>{
                \begin{itemize}
                    \item Data is partitioned using METIS
                    \item Replicate vertices using \emph{n-hop guarantee}
                    \item If the radius of $Q$ is not larger than $n$, $Q$ can be locally evaluated in each site 
                    \item If the radius of $Q$ is larger than $n$, $Q$ is decomposed into several subqueries $Q_i$ that can be independently evaluated; then their results are joined 
                \end{itemize}
            }
            \only<2>{
                \item Shape 
                \begin{itemize}
                    \item Generate triple groups based on common subject or object
                    \item Semantic hashing triple groups 
                    \item Allow some data replication between different partitions
                \end{itemize}
            }
        \end{itemize}
    \end{column}
    \begin{column}{0.55\textwidth}
        \centering
        \only<1>{
                \centering
                \begin{pgfpicture}
                    \pgftext{\pgfimage[width=5cm,height=3cm]{scratch1.png}}
                \end{pgfpicture}
        }
        \only<2>{
                \centering
                \begin{pgfpicture}
                    \pgftext{\pgfimage[width=5cm,height=2cm]{scratch1.png}}
                \end{pgfpicture}
                \vskip0.5cm
                
                \centering
                \begin{pgfpicture}
                    \pgftext{\pgfimage[width=5cm,height=2cm]{scratch2.png}}
                \end{pgfpicture}
        }
   \end{column} 
\end{columns}
        
\end{frame}
\end{document}