Regarding Observation 4304584 at the very bottom of the waterfall there is a signal appearing like it was just emited by the satellite.
It happened few times in the past that I was seeing similar waterfall.
Is it something wrong with Doppler shift calculation at the very end of the data stream (something in Gnu Radio processing chain) and that signal really belong to the brightest tangent line?
This is a frequency issue at the start of the observation, I remember discussing about it but I think that this is fixed in the latest version of the client. So, indeed the signal is part of the tangent line, you can also say that as the tangent line starts when this signal stops.