Thank you so much for this. It solved the issue for me on Ubuntu 20.04. Repeating for posterity:
Edit the clamav start up file
sudo nano /etc/systemd/system/multi-user.target.wants/clamav-daemon.service
Add to the end of the correct sections
[Unit]
...
JoinsNamespaceOf=apache2.service
[Service]
...
PrivateTmp=true
Restart services
sudo systemctl daemon-reload
sudo systemctl restart clamav-daemon.service