This isn't a bug in k3b or in any other CD burning program. Rather, it's a known side effect of enabling the tmpfs file system, in which the tmpfs file system "works as designed." To clarify the side effect, if you enable the tmpfs file system, your /tmp directory will exist in RAM, not on your hard drive. Once the /tmp directory in RAM has been filled, there's no more temporary space to use.
This topic has been discussed several times with the consensus being not to use tmpfs if you don't have to. ;-) While tmpfs might be great on a machine with loads of RAM, I don't recommend using it on most boxes.