Why does this sometimes not save the session history

.bash_logout

# ~/.bash_logout
cat "$HISTFILE" >> "$HOME"/.bash_history && /bin/rm "$HISTFILE"

bash_session_histories

SESSION_START_TIME=$(date +%F_%H.%M.%S)
mkdir -p "$HOME"/.bash_history_sessions
chmod 700 "$HOME"/.bash_history_sessions
HISTORY_SESSION_FILE=$HOME/.bash_history_sessions/$SESSION_START_TIME
HISTFILE="$HOME/.bash_history_$SESSION_START_TIME"
__save_session_history() {
  [[ -n "$HISTORY_SESSION_FILE" ]] && history -a "$HISTORY_SESSION_FILE"
}
PROMPT_COMMAND='__save_session_history(); $PROMPT_COMMAND'