I did not find online anyone mentioning about this. But after I update my S8 to Android Oreo (8.0, the latest system version), my Project Fi lost its data connections.
I found this issue after I left home taking an Uber. I was standing outside and there was no WiFi. My S8 behaved as usual until I notice there was no LTE sign in the status bar. I opened the Uber app and it said "No Internet Connections." I tried other apps and finally found my LTE was gone.
Okay, so long story short, if you encounter the same problem as I did, here is the solution. I do not know why it happens, but the update triggers a change in the network access point settings. For S8, if you navigate to
Settings > Connections >Â Mobile Networks > Access Point Names
You will see Project Fi -Tm is selected (there might be two of them). Click on the one that is already chosen and we'll edit the APN protocol. The update somehow changes the APN protocol to IPv6 only. Now change that to IPv4/IPv6.
Everything now gets back to normal. đŸ˜€