NVENC Video Codec SDK 8.2.15 SliceMode not working

This is my NVENC encoder config built from SDK

initializeParams.encodeConfig = &encodeConfig;
encGL = new NvEncoderGL(getWidth(), getHeight(), NV_ENC_BUFFER_FORMAT_ABGR);
encGL->CreateDefaultEncoderParams(&initializeParams,
NV_ENC_CODEC_H264_GUID,
NV_ENC_PRESET_DEFAULT_GUID);
encodeConfig.profileGUID = NV_ENC_H264_PROFILE_BASELINE_GUID;
auto codecConfig = &encodeConfig.encodeCodecConfig.h264Config;
initializeParams.frameRateNum = 60;
initializeParams.reportSliceOffsets = 1;
encodeConfig.gopLength = NVENC_INFINITE_GOPLENGTH;
encodeConfig.frameIntervalP = 1;
encodeConfig.frameFieldMode = NV_ENC_PARAMS_FRAME_FIELD_MODE_FRAME;
encodeConfig.rcParams.rateControlMode = NV_ENC_PARAMS_RC_CBR_HQ;
encodeConfig.rcParams.averageBitRate = 800 * 1000;
encodeConfig.rcParams.maxBitRate = 1200 * 1000;
codecConfig->idrPeriod = encodeConfig.gopLength;
codecConfig->repeatSPSPPS = 1;
codecConfig->sliceMode = 1;
codecConfig->sliceModeData = 1344;
encGL->CreateEncoder(&initializeParams);

I’m trying to slice the output to fix MTU size (1500bytes)
But the output is not sliced and I’m getting single slice with size > 1344 bytes. Can someone point me what is the problem here?