I've noticed on my Pixel 2 XL that with Nova, the swipe right gesture from the Home button (to toggle between the last 2 apps) doesn't work quite right -- a 3rd app often gets mixed in after maybe the 3rd or 4th swipe. When I switch back to the Pixel Launcher, it works fine, toggling only between the last 2 apps. I sent some feedback to Teslacoil about this. Check on your phone to see if the same thing happens to you.
It works fine now.
For me, it still throws in a 3rd app on occasion, instead of reliably swapping between the last two apps that were used.