I submitted a new block using the rpc command submit. And I just got a "reject" string without a reject reason.

I checked the bitcoind source code, and found that it usually sends a reject reason; bitcoind would only omit a reason if the strRejectReason is empty. Why and when would the strRejectReason be empty?

static Value BIP22ValidationResult(const CValidationState& state)
{
    if (state.IsValid())
        return Value::null;

    std::string strRejectReason = state.GetRejectReason();
    if (state.IsError())
        throw JSONRPCError(RPC_VERIFY_ERROR, strRejectReason);
    if (state.IsInvalid())
    {
        if (strRejectReason.empty())
            return "rejected"; // The value that gets returned
        return strRejectReason;
    }
    // Should be impossible
    return "valid?";
}

Source.