LastPass tries to be helpful by filling in form fields for you, but sometimes it gets it wrong. If you see it fill in the wrong data or if you just mysteriously struggle to sign into services that use 2FA (for me LastPass was helpfully inserting an old 2FA code when I tried to sign into AWS, which meant AWS thought my authenticator app was wildly out of sync and would make me resync it every time I tried to log in), follow these steps and see if it helps.
In case that link ever stops working, you’ve got to find the problem site in your vault, click the little wrench icon to edit that site, then click the little wrench icon again in the edit popup to edit form fields, then delete all the fields with bad values. There may be quite a few of these.
Credit where it’s due, one of my coworkers told me about this. Thanks Logan, I don’t know if I ever would have found that on my own!