Joint data is not sent if it doesn't fit in one packet.

[ksuprynowicz]

This happens completely without warning, causing joint data to not send at all and making animations stutter or not work at all.
It can also stops transmission of blendshapes sometimes.
A fix would be to instead send two or more separate packets (not NLPacketList) in such case, each containing partial set of updated bones.
I believe this would be pretty easy to do. A place to start is AvatarData.cpp:

    int maximumByteArraySize = NLPacket::maxPayloadSize(PacketType::AvatarData) - sizeof(AvatarDataSequenceNumber);

    if (avatarByteArray.size() > maximumByteArraySize) {
        avatarByteArray = toByteArrayStateful(dataDetail, true);

        if (avatarByteArray.size() > maximumByteArraySize) {
            avatarByteArray = toByteArrayStateful(MinimumData, true);

            if (avatarByteArray.size() > maximumByteArraySize) {
                qCWarning(avatars) << "toByteArrayStateful() MinimumData resulted in very large buffer:" << avatarByteArray.size() << "... FAIL!!";
                return 0;
            }
        }
    }

This part should generate more packets instead if I understand correctly.