Hi,
This does look to be a possible bug in MKFILTER. Fortunately, the code for MKFILTER is accessible. If you got to lines 60 and 62 of mkfilter.m and change them from:
sys = nd2sys(1,b1);
sys = nd2sys(1,b2);
respectively to:
sys = tf(1,b1);
sys = tf(1,b2);
Then you may be able to get the desired output that you are looking for.
~Zack