Some login credentials are not typical password inputs so I must leverage form fields to record them. I find form fields disappear in my vault from time to time without a clue. And the worse part is, there were reports back to 2018! You keep the passwords for me. It is the core feature you provide to the customers. Please address the issue. BTW, you know there is no form field history to trace back, right?
Could you try these steps to force a sync and hopefully resolve this issue?
Click the LastPass icon in your web browser toolbar.
Go to Account Options or click your email at the bottom of this menu > Advanced > Refresh Sites then Clear Local Cache.
Log out and log back in to your LastPass account and test again