ROG STRIX Z790-F GAMING WIFI II - Second GPU only getting 150 watt

[Uorufu]

Recently I had a power surge that broke my old motherboard (Asus prime z790-p), I replaced PSU and motherboard for a ROG STRIX Z790-F GAMING WIFI II (psu is now 1600watt Phanteks Revolt)

Anyhow, I'd tried pretty much everything, but my second GPU is only getting 150 watt in the lower PCI-e slot.
It gets 300 as normal if I put it in top slot. I also tested it on an old motherboard(ddr4 mobo I still had with an older cpu) and there it gets 300 watt proper in lower slot below a 3080 rtx just fine)

GPU-z shows second GPU is properly connected with PCIe 16x @ 4x as it should

Is my new motherboard bricked?
Things I tried:
- I updated bios to latest
- I did a clean reinstall of windows.
- Tried many different bios setting (different bifurcation settings, 16x, 8x 8x, gpu+m2).
- enabling/disabling resize bar
- manually setting PCIe settings.
- Many different nvidia drivers.
- Connecting the A6000 with either a single eps12v cable or 2x pcie-8pin to eps12v cable splitter

The RTX a6000 is fine, working properly in an older machine as well as in top slot, it just does not want to work in bottom slot, being perfcapped by power at 150w no matter what I do.

I think I am pretty much out of options. Is this an issue with ROG STRIX Z790-F GAMING WIFI II specifically and should I go back to buying a prime z790 instead? Or have I overlooked something....

edit: 
One thing I forgot to mention. It seems to only draw board power and ignore the connectors in bottom slot, which is as weird as it sounds. As it doesn't do this in any other slot.

[Jiaszzz_ROG]

Hello @Uorufu 

Thank you for reaching out to us. Based on your description, it seems that aside from the 150W power limitation on PCIEX16(G4), there are no other issues with usage.
Could you confirm if you directly installed the RTX 3080 in PCIEX16(G5), the RTX A6000 in PCIEX16(G4), and were any adapters or extension cables used?

How did you determine the current power output for the two PCIe slots? After clearing CMOS and running the system under default BIOS settings, is there any difference? 
Please share the following information to help us better understand your setup:
- The current BIOS version, OS version, and OS build
- The brand, model, and installation slots/ports for the CPU, GPU, RAM (including part numbers), SSD/HDD, and PSU
- Results or screenshots showing the power output for both PCIe slots and any other relevant details

Thank you.