This is OK. Imported shows just number of routes propagated to directly connected table, while preferred count shows cummulative number of routes originated by that protocol in all tables where these routes are preferred. Hm, personally I would prefer to receive information only about [some BGP protocol] when asking for information about [some BGP protocol] - this way I'll have 23857286917 preferred prefixes in no time, think about it yourself: how useful is that number? :)
So you removed it? Or renamed? I removed it, so another interface took it's interface id, but bird kept associating previous name (vlan's) with this id. - renaming fails in principally the same way: bird keeps previous name ;)
It is supposed to work (it is handled as removing and readding the interface), but probably not tested. Well, I was supposed to be awfully rich by now, but it wasn't tested, hence it didn't work :>