Blame ASUS (and every other PC laptop manufacturer) for not describing the GPU/display connection capabilities properly. Like I said before, many PCs only allow have one DisplayPort connection to a Thunderbolt port. I don't know if that's true in your case.
All Macs have at least two DisplayPort connections per Thunderbolt controller. The M1 Mac is weird in that it also has two DisplayPort connections to a Thunderbolt controller/port but they can both be used only when a tiled display like the LG UltraFine 5K or Dell UP2715K is connected.
M1 GPU can have two displays (a normal display or a tiled display can be connected to Thunderbolt, the other display is the built-in display or HDMI port in the case of the M1 Mac Mini).
Intel GPU can have 3 displays.
Nvidia GPU can have 4 displays (I think there's a trick that allows a tiled display to be connected as the last display - so that 5 total connections can be used in some cases).
AMD GPUs can usually have 6 displays.
What GPU is the Thunderbolt connected to? Maybe check the Device Manager, view by connection, find the display, and see what GPU is the parent device.
In your first post you said you had 3 monitors connected to the Thunderbolt 3 to Dual DisplayPort 1.2 adapter. How is that possible? Are you using an MST Hub or MST chaining? If the displays are connected to Intel GPU, including the built-in display, then you can only connect two external displays.
If your laptop has an Nvidia GPU, then why would the displays be connected to iGPU? I dunno. In the case of a Mac, the Intel GPU is used unless 3d acceleration is required or if external displays are connected. In that case, a MUX switches the internal display from iGPU (integrated) to dGPU (discrete). In a PC laptop, one possibility is that the dGPU will do rendering, but has to transfer the rendered content through iGPU to the display?
