So this sounds like the chip which controls the switching between internal and external monitors may be de-coupling from the board resulting in intermittent signal loss.
The thought is the flow is different than the external monitor thus you see it more on HDMI and when you are using the laptop screen it is sometimes getting confused.
I recently had a G752VW have the MiniDP and HDMI port fry (maybe the chip too?) but the USB C output as well as internal monitor still work. I went ahead and purchased a G752VSK-OC as a replacement but the old laptop still works.
With that in mind, you can try and utilize the USB C to Display Port or USB C to HDMI to your monitor as the BUS seems to differ compared to the HDMI/DP chip.
A more deeper dive is checking the chip (not sure which one) and seeing if any of the traces are NOT making good contact with the board. The chip very well could be going out as well (unlikely but still possible).
Based on your comfortability in opening the laptop, you can check the chips on the right side of the board, if not then I suggest utilizing USB C in the interim and see if it decreases.
Look forward to your response!
Model: ASUS ROG G752VSK OC Edition
Screen: 17.3’ 120hz TN 1080p Screen
CPU: Intel i7-7820HK 2.9/3.9GHz.
Memory: Kingston Fury 32 GB DDR4 3200 MHz. RAM
Graphics: Nvidia GTX 1070 8GB GDDR5
Storage:1TB Samsung 970 EVO PLUS NVME SSD (OS) + 500GB Samsung 960 PRO NVME SSD (Docs/Scratch Disk) + 1TB Samsung 850 EVO SSD (Torrents) + 500GB Crucial MX300 SSD (Photoshop)
NAS (n0x): 12TB Netgear ReadyNAS 104 (3)4TB HGST RED 7200RPM 128mb cache