How to remove things from File History?

Question

Background

I have the following configuration:

• File History backing up to drive D:
• Library X composed of one folder C:\X and one folder D:\X

My D: drive is growing full and I want to remove backups of D:\X from File History.

Symptoms

The first thing I tried was to add D:\X to the list of File History exclusions and select "Clean up versions" from the FH "Advanced Settings". The text describing this says:

The clean-up deletes versions of files and folders older than the selected age, except the most recent version of a file or folder. All other files and folders, such as versions that were excluded or removed from your libraries, are also deleted.

However, this does not remove the backups of D:\X from file history. They are still shown in the FH restore UI, the files containing those backups are still present on the drive, and the drive's free space has not increased by anything like the size of that folder.

The second thing I tried was more extreme: remove D:\X from library X (it is not part of any other library) and "Clean up versions" again. Once again, the backups are all still there.

The third thing I tried was more drastic again: delete library X altogether and "Clean up versions" again. Once again, the backups are still there.

After all this, I notice that I have an entry in the File History event log, with source FileHistory-Core, saying "Unusual condition was encountered during scanning user libraries for changes and performing backup of modified files for configuration [CENSORED]\AppData\Local\Microsoft\Windows\FileHistory\Configuration\Config". The event is at a regular FH backup time. It is from after I started trying to get D:\X removed from the history, but I am not sure at what point after that.

If I deliberately kick off a new FH copy operation, it doesn't do that again (though it does fail to copy some files, because the drive hosting my File History is full). And the files it's copying (or in some cases failing to copy) include ones from D:\X which it "should" no longer have any reason to be trying to back up.

The fourth thing I tried was to stop and restart the File History service, ask it to do another backup, and then "Clean up versions" again. (In case some of its configuration was in memory and not getting updated correctly.) Still no luck.

[EDITED to add:] The fifth thing I tried, because I needed to free up some space in a hurry, was: stop File History (this time I did it via the switch in the "modern" settings app), delete the folder inside D:\FileHistory corresponding to the D:\X folder I don't want backups of any more, restart File History. This successfully freed up a lot of space ... but File History has once again started backing up things from D:\X. So I think the underlying problem isn't so much "how to get rid of this stuff?" as "how to actually stop FH backing up a folder?", because it seems that even when I manually exclude the folder, remove it from the library that was causing it to be backed up, remove that library altogether, and blow away the folder where FH was storing its backups, FH just won't let it go.

[EDITED again to add:] One day later, it now appears that it's not backing up things from D:\X any more. I don't know what changed overnight; it certainly isn't anything I actively did during that time because I didn't do anything.

Questions

First, whatever is going on?

Second, what can I do to clean things up? My understanding (see Can I safely delete individual files from the Windows File History?) is that simply deleting the old FH files, while it won't exactly break anything, will leave File History thinking it has backups of them and trying to show them to me in its UI (and then restore operations will fail if I try to use them). That seems dirty. But what else can I do?

Probably-irrelevant notes

I'm on Windows 8.1. My D: drive is actually a RAID-1 mirror. The pathnames above are not exactly the real ones but they are correct in spirit. While the D: drive, on which the File History backups are stored, is full, the FH configuration lives in the location mentioned above, on another drive that is not full.