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M.2_2 slot runs at x1, missing settings in BIOS (Prime Z390-A)

Level 7
Hello everyone, since Asus customer support wasn't really helpful, I'm reaching out to you, hoping to resolve this issue or at least understand what's happening. The motherboard is Prime Z390-A, and the CPU is Intel i5 9600K.

I've installed 3 NVMe SSDs in the following disposition:
- M.2_1 - ADATA XPG, system drive
- M.2_2 - Samsung 970 Evo +, data drive
- PCIE x16_3 - Samsung 970 Evo+, data drive

They all run at expected speeds, except the M.2_2 slot which gets about 800 mb/s, or PCIE x1 (confirmed in the Samsung Magician app). The motherboard manual says something about M.1 configuration, but those settings just don't exist in UEFI under Onboard devices. Also in the UEFI, there is a "NVMe Configuration and Drive Info" section which has "NVMe Configuration" simply greyed out:

PCIE x16_3 shares bandwith with SATA 5 & 6, which are disabled and it indeed runs at full x4 speed. However, the manual clearly states that both M.2_1 and M.2_2 slots support PCIE 3.0 x4 bandwith as well. Is it possible that I have overloaded the bandwith or that it's being shared by some devices that I'm not aware of? Do maybe M.2_1 and M.2_2 slots share the same bandwith? I could have miscalculated available PCIe lanes, but the manual just doesn't suggest it. I've also attached the mobo diagram and the storage section from the manual if someone is interested:

This issue is nothing horrible, I'm just interested to know if I can find the cause of it. Thanks everyone! 🙂

Customer Service Agent
Hello, bionucular.

Could you please help to confirm the question below?
1. What is the current BIOS version?
Please return the BOIS to its default value first.
2. Please cross-test two of the Samsung 970 Evo Plus in M.2_2 and PCIE x16_3 to see if the bandwidth sharing issue persists.

Thank you.
sharing complete hardware info & software config details will help us better understand the issue 🙂