I can understand why it’s unexpected for you as a user. For me as a developer, I need to ask (honest question): How would restic detect that the files which were previously located in /original/path/mydata are the same that are now mounted at /usbdisk/mydata? Sure, we could try matching the file names and sizes and inodes and device IDs, but all that is only approximate and not exact. Therefore, at the moment, we err on the side of caution and always read data that we’re not sure where it comes from. That’s what users can (and should) expect from a backup program: Saving the correct data 
We’ve had several reports now with users reporting their surprise and non-understanding of why restic re-reads the files. Do you have any idea on how to improve this situation from a user’s point of view?