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Have you contacted AsRock? What do they say about this matter?
If this was an issue with the Intel® Rapid Storage controller, the M.2 slot wouldn't even work.
I received a response from ASRock today. They said:
The RAID configuration with PCIe slot cannot be supported. It can be only created by software RAID.
However, we haven’t tested with M.2 to PCIE adapter on PCIE slot with software RAID configuration.
It possible software RAID but not 100% guaranteed able to configure software RAID.