Currently the mempool isn't persisted across restarts. The historical argument against doing this is that it would make transactions immortal. Expiration now addresses that issue.

Without preserving the mempool node behavior changes drastically across restarts which complicates production use (e.g. mining overly small/low fee blocks) as well as development testing. The lack of retention also causes the loss of txn priority modifications made with prioritisetransaction which makes the API less reliable.

{
    string strStatusBar;
    string strRPC;
    string strGUI;
    const string uiAlertSeperator = "<hr />";
    if (!CLIENT_VERSION_IS_RELEASE) {
        strStatusBar = "This is a pre-release test build - use at your own risk - do not use for mining or merchant applications";
        strGUI = _("This is a pre-release test build - use at your own risk - do not use for mining or merchant applications");
    }
    if (GetBoolArg("-testsafemode", DEFAULT_TESTSAFEMODE))
        strStatusBar = strRPC = strGUI = "testsafemode enabled";
    
    if (strMiscWarning != "")
    {
        strStatusBar = strMiscWarning;
        strGUI += (strGUI.empty() ? "" : uiAlertSeperator) + strMiscWarning;
    }
    if (fLargeWorkForkFound)
    {
        strStatusBar = strRPC = "Warning: The network does not appear to fully agree! Some miners appear to be experiencing issues.";
        strGUI += (strGUI.empty() ? "" : uiAlertSeperator) + _("Warning: The network does not appear to fully agree! Some miners appear to be experiencing issues.");
    }
    else if (fLargeWorkInvalidChainFound)
    {
        strStatusBar = strRPC = "Warning: We do not appear to fully agree with our peers! You may need to upgrade, or other nodes may need to upgrade.";
        strGUI += (strGUI.empty() ? "" : uiAlertSeperator) + _("Warning: We do not appear to fully agree with our peers! You may need to upgrade, or other nodes may need to upgrade.");