Update this morning from support:
The issue you described was recently escalated to our developers, and it is under investigation now. While we do not have an ETA, please rest assured that our team is actively working on a solution in the most expedite way. We'd like to apologize for any inconveniences that this issue may cause you.
Pixel 4a, same issue
I was having the same problem with my Pixel 4a. I was able to fix this by removing my fingerprints and adding them again as outlined on this page: https://support.logmeininc.com/lastpass/help/why-isnt-the-unlock-with-biometrics-feature-for-lastpas...
AND...nevermind. This worked temporarily last night, but the issue has returned when logging into a website this morning. As stated in other posts, fingerprint recognition works fine when opening the app, but it is not recognized when the "fillable" pop-up appears.
Shucks. And here I thought we were good to go.