It's fairly common for iGPUs to have less outputs. Apple M1 was especially bad as it only had 2, and the internal screen on the laptops couldn't even be disabled if I remember correctly. I think many Intel (or maybe AMD) iGPUs only have three outputs.
Yeah it definitely sounds like a driver issue. I have had issues with dual GPU systems like that on Linux, not had any on Windows yet. It would be interesting to see to be honest. I've had laptops before where the video ports would only connect to the dGPU, and the internal screen used Optimus (display output from the iGPU with graphics acceleration from the dGPU on demand). Lots of dual GPU laptops are MUXless like that in fact.