So for me any reference not to start APU above FL 250 is only applicable to this specific scenario and has to do with fuel boost pumps not powered anymore
are there any other reasons to start the apu above 250 unless you lose both edg's?
i don't have that much experience yet but i can't think of anything else as you can't use bleed above 170
i looked it up in my manuals and couldn't find it as a limit anymore
it was probably somewhere during the ground course they told us