The situation is trying to get 5 instances of MH running across 2 4k monitors (with a 3rd unused) and going with a full screen on display 1, 4 quadrants on display 2, swapping, no hole left it seems ISBoxer has an issue with rendering the clients at 3840x2160 due to MH not supporting that res in the ingame options but requiring an ini edit to configure it.
The symptoms that make me think this is the case is that the single client on display one actually renders across display 1 and 3, the gui is maxed in size and not all of the screen is being rendered (eg it seems to be rendering through past the bottom of the screen) and the four quadrant clients render the same amount of screen (with increased GUI size) as display 1 shows but they are missing all the content that's also showing on display 3.
I'm assuming this is an untested edge-case caused by the hardware and the game not seeming to officially support 4k and I can record the 3 screens if you need more evidence or as an example.
I've also got all clients set to fullscreen windowed mode.
Thanks in advance, hopefully this is either user error in some spot but I've used it for years with wow and D3 or an easy fix on your end as I'm having more fun in MH than any other game in ages so would be nice to be able to box my non-main accounts