Are you connecting to the AVR using the optical out of the TV or the optical out of the Shield TV?
A splitter won't work at all because the Shield will see the AV receiver connected over HDMI and will drop the video signal down to a lower quality, or if it doesn't, you will still have a video signal your receiver can't work with.
You could try a HDMI audio extractor. It will need to be compatible with HDMI 2.0 and 18Gb/s HDMI, which will cost a bit more, but there are some out there...
This one meets the called for specifications, but read reviews carefully...
Amazon.com: SGEYR HDMI 2.0 Audio Extractor Optical 4K Converter HDMI Audio Extractor Splitter RCA(L/R) Stereo Analog Output HDMI Audio Adapter Support 4K@60Hz HDR HDCP2.2 EDID ARC for Speaker Amplifier Soundbar: Home Audio & Theater
www.amazon.com