I’ve encountered strange inconsistencies with Send A and Send B depending on mode. Post Fader mode has been particularly problematic.
If I have Send A set to post fader the amount of signal received by Return B is affected by how much Send A (and of course Send B) is turned up.
You can read more details of my testing here: DSP: Irregularities between Send A and Send B in Post Fader, Pre Insert, and Post Insert Modes.