Custom size image not shown to anonymous

I've got custom size images shown to the user, but for some reason the images stop working after some time, but ONLY for the anonymous users. The path is /system/files/images/2.img_assist_custom.jpg and I'm 19023% sure that image is there. When I log in and see the same page, the image works - then see the same page in a browser where I'm not logged in and again, only a marker of a broken pic. The image assist macro gets rendered and the -tag appears, but image is not shown.
This bug appears only when the cache is on. Cache is set to normal. Clearing the cache makes the images show again, but they break down again after some time all by themselves, without even anyone logging to the site.

I think this problem has been there for ages but I've always fixed it by adding another and another predefined image size, so that image will be created on the first time and then shown instead of showing the custom, but I wouldn't wanna use this fix over and over again since it makes so many of them already and this clearly is a bug.