I have a test set up that can more or less be broken down like below:
PCB1A + PCB2A pass the 12V line through to the Mini-ITX CPU board, which can be powered by 12V directly via the 4 Pin ATX connector; it does not need the full 24 pin ATX connector. PCB1A and PCB2A each have a PMOS to allow cutting power to the Mini-ITX, however they do not condition the power at all, i.e no regulation, boosting, etc.
I notice that if I power cycle Mini-ITX-B using PCB1-B, Mini-ITX-A also power cycles as Mini-ITX-B comes back online. Probably due to a voltage spike or dip.
I do not notice this with any other combination of assemblies, i.e. A-C, B-C, A-D, B-D, etc.
Notice that A+B are both tapped off of 1 output of the splitter, and are the shortest cables. I'm guessing this has something to do with it.
Does anything obvious jump out as to why this is happening? And, hopefully, is there an easy fix (i.e. a big cap, or ferrite bead somewhere)?