Perhaps LastPass sets a hidden form field incorrectly thinking that it knows better than XenForo what that field should be set to. In any case, I could not get it to accept the password as correct by pasting it or autofilling it. Problem is, I had just used that same password to log in the normal way.
Turning off LastPass allowed me to defeat this oddity and finish associating that account with Facebook, but I would like to investigate why. (is this something I should report here or to LastPass?)
It could just be a one-in-a-million-chance thing because I am running around testing with multiple accounts for that domain saved into LastPass - I don't know if it modifies a hidden input field which then invalidates the password I wanted to paste into it.