I'm not sure what troubleshooting you've tried so far, does uninstalling the LastPass app, restarting your device, then reinstalling resolve this issue?
Thanks for giving that a try. Could you try these steps out?
Disable autofill oreo and legacy in your device's autofill settings then restart the device
After that, enable autofill oreo only and test
If the issue persists, disable autofill oreo and then enable autofill legacy only, then test once more
Our development team is actively working to resolve issues with biometrics not working as expected with the Android app. So that our team can gather some more specifics and keep you updated, could you submit a bug report following the steps here? https://support.logmeininc.com/lastpass/help/how-do-i-report-a-bug-to-lastpass