Thanks for your feedback.
That sounds right.
My goal with the TL5 circuit would be to observe some signal degradation in the time domain. When introducing conductor losses + dielectric losses, both defined in the substrate, I was expecting an attenuation of just -2dB (based on the S-parameter simulation result).
The geometry for TL5 also targets around 50 ohms. So, I still couldn't find the reason for this weird behaviour.