I would wonder if it is because of the BIOS. Could you test booting maybe Linux or Windows from a USB drive to see what happens and if they can boot and see the full RAM? Though I suspect they might be able to overcome whatever limitation from the BIOS which Haiku is unable to overcome. Or maybe the way the extra RAM is added needs special support from the OS which Haiku does not have.
It does seem odd that Haiku won’t even boot, rather than just ignore maybe the extra RAM it does not support.
I tried searching some online and did not see too much about this, except people who said the BIOS also did not see the extra RAM, which is not your problem.
I am not much help, maybe some other Haiku developers with more kernel experience will have ideas.