Straight ML-TLs are two summed resonant structures of which one is a 1/4 WL and the other a 1/2 WL resonator, so there's an unbalanced number of both even and odd order harmonics, ergo the system is non-harmonic in its passband, comb filtering with the driver's harmonics, screwing up its response until the vent's output has decayed to ~ -25 dB.

GM