You're not going to get it to work via Internet Recovery seeing as that literally downloads the OS from the Internet directly to RAM. I'd imagine that you can't change it for the recovery/preboot environment for anything newer than Catalina since the OS is effectively immutable except for changes made by Apple. I think if you could change it, it would probably be a massive endeavor.
I’m waiting to hear back from Apple as to whether it’s a modifiable or even controllable behaviour.
As of the logic behind which display is used for these things, my guess is that the ports on your video card are assigned numbers and that Apple defaults to whatever is in the number 1 port.
It’s not that as far as I can tell, as it’s ignoring my MPX bus 0 port 1 display, to put the preboot on bus 1 port 1.
I'm just guessing on that one. If that changes with each boot, then I'm not sure what to tell you. Generally speaking, though, on Intel Macs, those preferences as to which display is your primary vs. secondary are stored in PRAM. I'd imagine that at least the preference as to which display is first would be saved in PRAM, though the recovery environment is definitely not sophisticated enough to recognize a vertical display layout, which is why you have it sideways.
Interesting to see that safe mode only seems to recognise 2 displays total, and by default will display to my two side displays, ignoring the middle. If I unplug a side display, the middle is brought into play.
It’s not just that it’s not displaying on them, but third displays simply don’t show in system report. You also don’t get rotation in safe mode.
I’m inclining towards thinking the middle display is slower to initialise its displayport connection, so the 2 slots are being grabbed by the side displays before the middle is out of bed.
