In the FFX spec, there is a note about using CMAC as the round function.
Security notes. The round function F is constructed in such a way that the set of inputs on which the CBC-MAC is invoked is preﬁx-free. (A set of strings is preﬁx-free if for any distinct x, y in the set, x is not a preﬁx of y.) The CBC-MAC is known to be a good PRF when it is invoked on a set of preﬁx-free inputs, assuming AES is a good PRP .
Why is it important that the input be prefix-free? The citation is for this paper on CBC-MAC by Petrank and Rackoff.