Your browser's built-in password manager likely is overwriting the fields. Check the saved entries in it, and delete anything LastPass related.
Hello @matthew.ross,
Thank you for checking the Community!
As @jepenny mentioned, this is related to the browser's built-in password manager. Please, review those settings as they might be interfering when you are using the vault via the web. We recommend you use the LastPass extension, you may download it here.
Let us know if further assistance is required.
Thank you,
Karol | Customer Care