I have the same problem.
My setup is unRAID running tvheadend docker, with Hauppauge WintV-Quad being passed through.
When the WinTV-Quad is passed through to a Windows VM, the Hauppauge WinTV software detects all channels, but tvheadend doesn't detect 2 channels (one channel SNR 35dB, Signal Strength -22dBm and the other channel SNR 25db, -45dBm)
Does tvheadend (or maybe the v4l stack) need to reduce it's thresholds?
I have also had this issue with other devices where it tunes fine in Windows but tvheadend doesn't get all channels...