Phillip Corcoran :
"It doesn't matter whether the internal graphics (IGPU) is enabled or not.
The monitors will always work no matter what how you connect them".
That my be your experience mate but it depends on the actual mobo.
On my 6-year old ASRock board, iGPU must be manually disabled in BIOS, otherwise PCIe GPU will not work.
So don't assume all boards and BIOS behave in the same way 'cos they don't.
Ok. I'm with You
On the Asus mobo in question with UEFI BIOS it is per default Auto, meaning that if a PCI-e card is mounted it will first try to use this, but as can be seen on the picture above from my Asus Z97 Deluxe mobo (which i forgot to link to before) it can also be set from auto to either IGX or PCIE or the option below to be totally disabled.
On the ASUS STRIX Z270F GAMING BIOS
Manual page 41
If this is the case I would recommend leave it enabled and in auto mode, because if something happens to the discrete PCI-e card, just unmount that and temporarily plug the monitors to the mobo connectors instead (perhaps via an DVI-HDMI/DP adapter).
Btw. Do you have the latest drivers installed?
https://www.asus.com/Motherboards/ROG-STRIX-Z270F-GAMING/HelpDesk_Download/
Best regards from Sweden