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 (State(pto->GetId())->fHaveWitness || !IsWitnessEnabled(pindex->pprev, consensusParams)) {
                    uint32_t nFetchFlags = GetFetchFlags(pto, pindex->pprev, consensusParams);
                    vGetData.push_back(CInv(MSG_BLOCK | nFetchFlags, pindex->GetBlockHash()));
                    MarkBlockAsInFlight(pto->GetId(), pindex->GetBlockHash(), consensusParams, pindex);
                    LogPrint("net", "Requesting block %s (%d) peer=%d\n", pindex->GetBlockHash().ToString(),
                        pindex->nHeight, pto->id);
                }
            }
            if (state.nBlocksInFlight == 0 && staller != -1) {
                if (State(staller)->nStallingSince == 0) {
                    State(staller)->nStallingSince = nNow;
                    LogPrint("net", "Stall started peer=%d\n", staller);
                }
            }
        }
        
        
        
        while (!pto->fDisconnect && !pto->mapAskFor.empty() && (*pto->mapAskFor.begin()).first <= nNow)
        {
            const CInv& inv = (*pto->mapAskFor.begin()).second;
            if (!AlreadyHave(inv))
            {
                if (fDebug)
                    LogPrint("net", "Requesting %s peer=%d\n", inv.ToString(), pto->id);
                vGetData.push_back(inv);
                if (vGetData.size() >= 1000)
                {
                    pto->PushMessage(NetMsgType::GETDATA, vGetData);