Michael Buckley:

macOS only allows sandboxed apps to access a limited number of files at a time, but there is no way for an app to query how many files it can open, or if it’s close to the limit. In fact, this limit is dependent on the amount of RAM installed in the computer and the number of files open by other apps. Because this limit only affects apps that can batch process thousands of files, and users that want to do so, many users—and developers—remain unaware of it, despite the fact that it has affected some of Apple’s own apps.

This is the primary reason Retrobatch isn't sandboxed (and thus, not on the App Store). I've considered just dealing with the likely support emails (and suggesting folks give it full disk access, or only add folder), but I just can't bring myself to do it. What a pain this bug has been for years.

Every year at WWDC, since about 2015, I would hunt down a particular engineer I knew who was aware of this issue, and the first question I would ask is if this bug was fixed.

See also this really old radar: rdar://13006144: Sandbox and limitation on number of files that can be opened from a user.