I also thought about the possibility that the UEFI to OS handover goes wrong, resulting in a kernel panic, but I'm quite sure the issue will also occur when booting without any displays attached (in this case the EFI display driver shouldn't do anything).
Could confirm this thoughts, cause i made the following tests:
As followers of this thread know, i have a AMD R9 280x and HD5xxx and HD7970 to test in conjunction with my latest RX460 and RX480 Sapphire cards. So i tested them with AMD R9 280x (HD7970) in slot#1, RX460 (RX480) in slot#2. Have to mention, that R9 280x and HD7970 have two VBIOS modes, so i flashed them with one VBIOS including GOP, other VBIOS w/o GOP.
All owners of RX4x0 cards should know, that there is no VBIOS w/o GOP code, so we have no "legacy only" VBIOS for these cards.
So first i used GOP-free VBIOS on both R9280x as well as on HD7970.
Set PCs BIOS to CSM enabled and VBIOS to "legacy": booted just fine, RX cards get accelerated.
Set PCs BIOS to CSM enabled and VBIOS to "UEFI": booted just fine, cause R9 280x or HD7970 in slot#1 do NOT have GOP/UEFI code in VBIOS, RX cards get accelerated.
Set PCs BIOS to CSM disabled and VBIOS to "legacy": booted just fine, RX cards get accelerated.
Set PCs BIOS to CSM disabled and VBIOS to "UEFI": booted just fine, cause R9 280x or HD7970 in slot#1 do NOT have GOP/UEFI code in VBIOS, RX cards get accelerated.
Switched VBIOS of R9 280x and HD79070 to GOP coded and tried again:
Set PCs BIOS to CSM enabled and VBIOS to "legacy": booted just fine, RX cards get accelerated.
Set PCs BIOS to CSM enabled and VBIOS to "UEFI": booted into BlackScreen, cause GOP code was used for Bootsequence
Set PCs BIOS to CSM disabled and VBIOS to "legacy": booted to BLACK SCREEN, cause CSM disabled always loads GOP code from card if availlabe.
Set PCs BIOS to CSM disabled and VBIOS to "UEFI": booted to BLACK SCREEN, cause CSM disabled always loads GOP code from card if availlabe.
Now same test with RX cards in slot#1 and IGPU as primary GFX-card:
CSM enabled or disabled makes no difference, cause HD530 seems not to have any GOP code in its VBIOS, so booting was always successfull with full acceleration of RX cards cause "legacy" code was used for bootsequence.
You could verify that your HD530 will have no GOP-code by running GPU-Z under WINDOWS. Here you won't see any checkmark for UEFI feature.
Sorry, currently i am at work, so i dont have any screenshot to show what i mean, but i will make a detailed report, when i am back home.
So what i think makes a solo installed RX4x0 card to boot to black screen, is: cause there gets always the GOP code of VBIOS loaded - no matter, what kind of settings you select within your BIOS setup. I know, that RX4x0 cards do have "legacy" and "GOP" code in it's VBIOS - but i dont know, how to disable the GOP-Code while booting system with this card "one-and-only" gfx card in your hardware.