GarbageCollector not removing all images

[StefanGla]

Hello,

I'm using the GarbageCollector as described:
GarbageCollect::dropOldFiles($this->img_cache_dir, 30, true);
where $this->img_cache_dir is my /web/image_cache directory.

The GarbageCollector deletes some files, but when I check one subfolder of /web/image_cache/0 for example, there are still older images in it:

2019-04-11+18:55:51.2625899500 ./web/image_cache/0/e/0/b/6/0e0b65c7624be7a5d48590b099c3290c9de97008.jpeg

I'm running the script as root, so there should no file access permission issues.