@lakk I have had to work (deal) with them from time to time. I can tell you I did the exact same thing with them (breaking the mirror) by (assuming) the intel raid controller acts like a traditional raid controller. I can tell you it does not, because it exposes both the raid device and the JBOD disks to the OS. The OS needs to be smart enough to know how to manage the array.
I did write a tutorial on how to use FOG with these type of raid adapters here: https://forums.fogproject.org/topic/7882/capture-deploy-to-target-computers-using-intel-rapid-storage-onboard-raid (oh my all the way back in 2016…)
I can tell you another example (possibly of what you are seeing). We have several dell precision rack mount workstations that use these raid controllers for their local disks. Somewhere in 2018-2019 they upgraded the OS from Windows 7 to Windows 10. About 6 months later we got a call that 2 of the workstations had reverted back to windows 7. This wasn’t possible because it was a clean install of windows 10 and not an upgrade from Windows 7. Its just not possible to do what they said it did. We had them reboot the workstation and take a few screen shots. They called back and said that it switched back to windows 10. Thinking they were just crazy we said the next time it happened give us a call. About a month later it did it again. To no make this any longer of an example I’ll cut to the point. We found that the raid-1 mirror was split (akin to split brain) some time before windows 10 was installed. So not knowing the mirror was broken they installed windows 10 and it went onto one disk while the other disk remained at windows 7 install. It appears that the intel raid controller picks at random which disk will be the leader and the other the follower in the mirror (for the intel controller the leader disk has read/write activity, while the follower only has write activity). That is how on one boot it would start up as win10 and the another boot win7.