Hi,
what you see is normal and expected. This is due to the nature of Transmission Lines (TL). TL provide the admittance/reactance only at the centre freq that you design it at. But generally the performance of TL and lumped filters are similar in the passband.
Also note that TL filters are periodic, unlike lumped, since L=lamda @ f1 becomes L=2lamda @2*f1....
Hope this answers you initial doubt. Find out more in filter books like "Lancaster", the ebook is found somewhere is this forum.
Cheers!