Cubase leaving unused files locked

(Cubase 11 Pro)
I’m writing music for a video and I’ve gone through several iterations of the video file. Each one I add to my project as a video track and when I decide to switch to the next version of the video file I remove the current one via selecting it and doing “Remove selected tracks” before replacing it with the latest iteration. I’m up to iteration number 5 now.

These are all MP4 files, which take up a lot of space so just now I tried to delete the first four, since I’m not using them for anything, but when I tried to delete them in Windows File Explorer I got a message that “The Action Can’t Be Completed Because the File is Open in Cubase 11

Why is Cubase locking files that are no longer used in the projects and how do I force Cubase to let go?

Thanks in advance.