Currently it is possible to stupidly lose mempool data by calling the rpc "savemempool" before the previous stored mempool has been read and processed to the memory mempool.

I haven't looked at the code yet, but at minimum one ugly solution would be to have a global g_is_mempool_read_finished (or something of the short) that starts false and gets sets to true once the reading is reading processing of mempool.dat is successfully finished. The rpc savemempool checks this global on the start and returns a an error while it remains false.

private:
    bool ActivateBestChainStep(CValidationState& state, const CChainParams& chainparams, CBlockIndex* pindexMostWork, const std::shared_ptr<const CBlock>& pblock, bool& fInvalidFound, ConnectTrace& connectTrace);
    bool ConnectTip(CValidationState& state, const CChainParams& chainparams, CBlockIndex* pindexNew, const std::shared_ptr<const CBlock>& pblock, ConnectTrace& connectTrace, DisconnectedBlockTransactions &disconnectpool);
    CBlockIndex* AddToBlockIndex(const CBlockHeader& block);
    
    CBlockIndex * InsertBlockIndex(const uint256& hash);
    void CheckBlockIndex(const Consensus::Params& consensusParams);
    void InvalidBlockFound(CBlockIndex *pindex, const CValidationState &state);
    CBlockIndex* FindMostWorkChain();
    bool ReceivedBlockTransactions(const CBlock &block, CValidationState& state, CBlockIndex *pindexNew, const CDiskBlockPos& pos, const Consensus::Params& consensusParams);
    bool RollforwardBlock(const CBlockIndex* pindex, CCoinsViewCache& inputs, const CChainParams& params);
} g_chainstate;
CCriticalSection cs_main;
BlockMap& mapBlockIndex = g_chainstate.mapBlockIndex;
CChain& chainActive = g_chainstate.chainActive;
CBlockIndex *pindexBestHeader = nullptr;
CWaitableCriticalSection csBestBlock;
CConditionVariable cvBlockChange;
int nScriptCheckThreads = 0;
std::atomic_bool fImporting(false);
std::atomic_bool fReindex(false);
bool fTxIndex = false;
bool fHavePruned = false;
bool fPruneMode = false;
bool fIsBareMultisigStd = DEFAULT_PERMIT_BAREMULTISIG;
bool fRequireStandard = true;
bool fCheckBlockIndex = false;
bool fCheckpointsEnabled = DEFAULT_CHECKPOINTS_ENABLED;
size_t nCoinCacheUsage = 5000 * 300;
uint64_t nPruneTarget = 0;