Okay, so I uninstalled MC24. Twice.
The first time I didn't select the "Remove Media Center Library Files" option, as shown in the attached image. The uninstall process left everything related to MC24 in the "C:\Users\[UserID]\AppData\Roaming\J River\Media Center 24\" directory, by the look of it. All 1.24GB for me, mostly in the Cache and Thumbnails sub-directories.
Then I reinstalled MC24, restored one of its Library Backups, checked the directory was still about 1.24GB, and ran the uninstall again, this time with the "Remove Media Center Library Files" option selected. Everything was removed from the "C:\Users\[UserID]\AppData\Roaming\J River\Media Center 24\" directory. The directory itself was left in place, but empty. Maybe I had the directory in use in Windows, so it couldn't be deleted. I'm not sure on that as I had Windows Explorer windows open, and hadn't rebooted during this testing. Note that I had to test this option as I don't usually use it myself, since I run multiple versions and Beta software regularly. I manually delete the previous version's directory when I am ready.
So, if you installed MC25 first, without uninstalling MC24, got MC25 working correctly, then uninstalled MC24 with the "Remove Media Center Library Files" option set, nothing would be left behind in the "C:\Users\[UserID]\AppData\Roaming\J River\Media Center 24\" directory. The "C:\Program Files\J River\Media Center 24" program directory was also completely removed. The "C:\Users\[UserID]\Documents\JRiver\Media Center 24" remained, but contained only the "Library Backups" sub-directory and the backup files, which is good.
So, bottom line, your original issue was a user error. While you aren't prompted to remove the previous version's files, you are given the option to do so. Maybe the wording of the option could be clearer.
I guess next time you will select that option. Just don't use that option before installing the next Upgrade version, unless you have a very recent Library Backup ready to Restore after the installation. Theoretically, by installing the new Upgrade first, the installation process will find the previous version and transfer the Library across to the new version. Sometimes that doesn't work if you still have multiple versions of MC installed, or if you use a Library that isn't in the default location. But for normal users who have only one version of MC installed, and use the Main Default Library in its default location, this would work.
Cheers.