This is the first I've heard of that problem, but given how much of a pain it was to implement that feature, it wouldn't surprise me at all to learn there's still a bug or two with it.
I'm at work for the next several hours, but when I get home I'll be able to take a closer look. If you could verify if the normal attacks are actually using the stats of the locked weapon, by slapping wildly unrealistic ATK values on them, that'll save me a few minutes in tracking down the issue though. If not, no big deal, I'll have a look when I can.