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.

{
    std::vector<unsigned char> commitment;
    int commitpos = GetWitnessCommitmentIndex(block);
    bool fHaveWitness = false;
    for (size_t t = 1; t < block.vtx.size(); t++) {
        if (!block.vtx[t].wit.IsNull()) {
            fHaveWitness = true;
            break;
        }
    }
    std::vector<unsigned char> ret(32, 0x00);
    if (fHaveWitness && IsWitnessEnabled(pindexPrev, consensusParams)) {
        if (commitpos == -1) {
            uint256 witnessroot = BlockWitnessMerkleRoot(block, NULL);
            CHash256().Write(witnessroot.begin(), 32).Write(&ret[0], 32).Finalize(witnessroot.begin());
            CTxOut out;
            out.nValue = 0;
            out.scriptPubKey.resize(38);
            out.scriptPubKey[0] = OP_RETURN;
            out.scriptPubKey[1] = 0x24;
            out.scriptPubKey[2] = 0xaa;
            out.scriptPubKey[3] = 0x21;