That's definitely a bug. Mapped addresses should never hit the wire.
Dual stack is quite a bit safer than NAT64/DNS64. The bug you describe should be fairly trivial to get fixed if someone can isolate which product actually has the bug. Have you tried the current kernel under the existing other components? If swapping the kernel doesn't fix it (I think the mapped address on the wire bugs in the Linux kernel were removed fairly early in the 2.6 chain IIRC), then it's probably Java.
Owen
It was a fairly recent kernel (2.6.31-19 #56 Ubuntu) and uptime on the machine I was testing on is a bit less than a year so it hasn't been updated in a while. I will try it again once that machine gets updated. I have seen a few bugs in various Ubuntu kernel builds, too. Such as in one build ND is broken (machine responds to its own DAD probes so it thinks any address it tries to use is in use) but the previous build and subsequent build (all of the same kernel version, just different patch tinkering by Ubuntu) work ok.