HFSS : different results between waveports and lumped ports

hello everyone

I read different posts about how to design wave ports and the difference with lumped ports but i haven't found the solution to my problem :
I currently work on a CPWG structure i am used to simulate with wave ports
(i have already checked that they are well designed)
but I was told that this definition of ports creates an artificial link between the coplanar grounds and the ground plane below (they are indeed already connected but with further metallic vias)
and this link is not representative of the real structure

so i tried to simulate with lumped ports : i connected the coplanar grounds with an U shape ground and in the middle i drawed a rectangle between the gnd and the trace, the same size of the middle trace and i assigned it as a 50-ohm- "lumped gap source" with impedance and calibration lines

and the S Parameters were totally different !
we have more reflexion in this last case and the transmission is worse (on the frequency range 0.1-20 GHz, in the three modes of simulation fast, interpo and discrete)

so the question is : WHY ?!
aren't we supposed to get similar results with these two kind of ports ?
and which one is reliable ?

thanks in advance for your answers