I also had the issue with the super-hot fingerprint reader until I upgraded libfprint to v1.94.0 or greater. I was wondering if it was a feature of the reader to erase your fingerprint from your finger 
Anyway, the temperature of the power button and the surrounding area are about the same now, so I am happy. In case anyone has installed from git clones and is not sure how to upgrade as new tagged versions of libfprint and fprintd come out, I made a script:
#!/bin/bash
cd libfprint/
git fetch
latestTag=$(git describe --tags `git rev-list --tags --max-count=1`)
echo $latestTag
git checkout $latestTag
sudo chown $USER.$USER -R .
meson setup --wipe builddir
sudo ninja -C builddir install
cd ../fprintd/
git fetch
latestTag=$(git describe --tags `git rev-list --tags --max-count=1`)
echo $latestTag
git checkout $latestTag
sudo chown $USER.$USER -R .
meson setup --wipe builddir
sudo ninja -C builddir install
cd ..