What happened to the money multiplier after QE?
You may want to look at the following paper by Cukierman (2017). The author compares inflation for a period of identical base expansion in post Lehman US and hyper-inflationary Germany in the 1920s. They mention exactly what you said, that banks have increased their reserves at the Fed. In the US this has not translated into a heightened demand for goods and services as was the case in Germany. Furthermore the author cites fundamental differences in the economical and institutional circumstances as a reason for the difference in transmission. I suggest reading the paper for a more in depth explanation.