I've been a Roboform user so my terminology comes from that background, apologies for the lexicon.
I have a site at work that uses RSA two factor authentication. The last field on the form is a four digit constant PIN followed by the RSA six digit "random" number - in the same field. I want the form filled but not submitted as those last six digits change every few seconds. I can find a way to not complete the form at all. I can find a way to complete the form and submt it. But I can't seem to find that halfway house. I've even read the manual. It's the only site, from hundreds, for which Roboform works and LP does not. It's also a modestly complicated form that, should I foul it up, locks me out. So completion would be really handy.
Aggravatingly, my bank site wants a bank account number plus a password - and then asks for different characters from my password. This isn't dissimilar - I want the form filled and control handed to me. That works. I speculate that the difference is that in the working case LP can sense that there is more information required as more input fields are present. Whilst, in the failing case, it sees a field and fills it, not really knowing that I have to tack some stuff on the end of that field.