Windows Storage Space file inconsistent?

Question

It is very odd.

1. I use Win 10 pro, storage space, 3*2T drives, Two-way mirror, mounted as D:
2. I created new folder in the root and moved another folder into it, i.e. D:/a/ becomes D:/new/a
3. I shutdown the machine properly by clicking shutdown
4. I removed the windows 10 SSD and swap to another windows 10 SSD (label this win10b)
5. Start the machine, sees D:/a/ coming back, able to read one of the file (means successfully mount an ISO in the folder)
6. Properly shutdown, swap back the original SSD, D:/new/a comes back
7. Swapped a few more times (step 4-6), same behavior
8. On win10b, disk management, remove drive letter and add it again, fixed

What is wrong?