in v0.12 https://github.com/bitcoin/bitcoin/pull/7081 (by @luke-jr) was merged adding `-bytespersigop` (with default of 20).

this is preventing simple bare multisig transactions from being accepted into mempool when the size is < 400 bytes.  
eg; https://gist.github.com/rubensayshi/ac4f617207f7a50559e85d61c05800be

from 1-of-7 and onwards they are accepted because the ratio of bytes:sigops is high enough.  
also having 2 or more inputs will work because it again tips the ratio of bytes:sigops over.

the reason mainly being that `GetLegacySigOpCount` calls `GetSigOpCount` with `fAccurate=false`.

It looks like changing `GetLegacySigOpCount` to use `fAccurate=true` would cause side effects in the few other places (`ConnectBlock`, `CheckBlock`, `CreateNewBlock`) and simply adding `fAccurate` as arg to `GetLegacySigOpCount` would affect the value stored in `CTxMemPoolEntry.sigOpCount` which in turn affects a bunch of other things.

if (filterrate && txinfo.feeRate.GetFeePerK() < filterrate) {
                        continue;
                    }
                    if (pto->pfilter && !pto->pfilter->IsRelevantAndUpdate(*txinfo.tx)) continue;
                    
                    vInv.push_back(CInv(MSG_TX, hash));
                    nRelayedTransactions++;
                    {
                        
                        while (!vRelayExpiration.empty() && vRelayExpiration.front().first < nNow)
                        {
                            mapRelay.erase(vRelayExpiration.front().second);
                            vRelayExpiration.pop_front();
                        }
                        auto ret = mapRelay.insert(std::make_pair(hash, std::move(txinfo.tx)));
                        if (ret.second) {
                            vRelayExpiration.push_back(std::make_pair(nNow + 15 * 60 * 1000000, ret.first));
                        }
                    }
                    if (vInv.size() == MAX_INV_SZ) {
                        pto->PushMessage(NetMsgType::INV, vInv);
                        vInv.clear();
                    }
                    pto->filterInventoryKnown.insert(hash);
                }
            }
        }
        if (!vInv.empty())
            pto->PushMessage(NetMsgType::INV, vInv);