Grub only boots working Windows. You should always be able to boot Windows directly from UEFI boot menu, F12.
But changing from RAID to AHCI means you need to add AHCI drivers into Windows. My Dell worked with RAID on, and did not need change to AHCI as it only has one NVMe drive and Ubuntu installed Intel® VMD driver. from lspci & SATA mode:
Kernel modules: vmd, ahci
Ubuntu installation on computers with Intel(R) RST enabled &
Intel RST
Windows AHCI instructions - some have found safeboot method better
https://www.dell.com/community/Laptops-General-Read-Only/Dell-M-2-FAQ-regarding-AHCI-vs-RAID-ON-Storage-Drivers-M-2-Lanes/td-p/5072571 &
https://askubuntu.com/questions/1233623/workaround-to-install-ubuntu-20-04-with-intel-rst-systems &
But if you do a safe boot first to update Windows, then boot to UEFI/BIOS and change to AHCI and finally boot normally, it works