Could you try these steps to see if that fixes this issue? In the app, tap Settings > Actions > Refresh Vault & Clear LastPass Cache. Log out and log back in. If that doesn't work, try uninstalling the app, restarting your device, then reinstalling.
I did that, and it seemed to have worked, but when I went to the site in question today, my lastpass vault had reverted back to the old value again. The whole point of lastpass is that I don't have to remember all of my passwords. If I can't trust it to remember changes, then why am I paying for it?