whamel, and EG
Oops, I didn't notice the 2nd QAM signal had gone up in your changed setup. Not sure, then, why the higher upstream frequency was attenuated and not the lower.
I found the manufacturer's spec sheet for these things and it seems that there is some small attenuation back for frequencies below 65 MHz of 1 to 1.5 or 2.0 dBmV, but at higher frequencies they have the labeled attenuation. That is, a 6 dBmV one will attenuate 6 to 7 dBmV at 88 MHz and higher. Generally, that would be OK for internet frequencies - upstreams are 25-36 MHz, downstreams are 580-606 MHz on my modem - but maybe not the higher frequencies used for cable TV since there's no data for >860 MHz. This suggests that these things should be plugged in to the cable input of the modem and definitely not at the input to a splitter that my connect eventually to a cable box.
I found a UK blog that shows these attenuators attenuate both up and down streams:
These results leave me with some questions about how well these things work.--
There is nothing more deceptive than an obvious fact.
Sherlock Holmes in
The Boscombe Valley Mystery
A. C. Doyle
Strand Magazine, October 1891