Looks like you may have found a bug when a user tries to 'switch' their Facebook account to a different Joomla user. Not a common situation, but definitely happens and should work.
The fix is very simple, if you're up for making a minor code change. To do so, please edit the /administrator/components/com_jfbconnect/models/usermap.php file. Around line 309, you'll see:
$row->load(array('j_user_id' => $jUserId, 'provider' => $provider));Change that to:
$row->load(array('provider_user_id' => $providerUserId, 'provider' => $provider));Let me know how that goes. We have a v5.1.2 release due out tomorrow with a few other bug-fixes, but I'm going to try to streamline this into our test suite and get it into that release as well.
Thanks for notifying us of the issue and describing the steps in such detail.
Alex