I noticed on my Google Pixel 2 XL, any app installed via apk file as opposed to Google Play store will immediately quit after I start it. Any idea wjy this is happening and how to fix it?
It's depending on which app you are installing.. whether they are giving support to Android Oreo 8.0 or not. You can realize that other standard apps like WhatsApp or Facebook from another market will work fine by installing manual apk.
May be the Google's certification authentication. to fix it, try to force stop google play store on settings and install the apps again.
I hope it get useful to you.