@tfry. I have confirmed that this is a known issue regarding Joomla code since version 3.6. I tested on a Windows IIS server, and confirmed that folderlist doesn’t work properly on a Windows-based web server. However, the issue is with Joomla, not my or anyone else’s module/extension.
I made a pull request on the Joomla git repo: https://github.com/joomla/joomla-cms/pull/12841
Apparently there were at least two other pull requests before mine, although their solutions were different: