Digital Light & Color

If you are asked to Use a replacement file (because the source image is not available for whatever reason), the loading of the tree is not cancelled, but goes on until the whole tree of errored red thumbnails is completed.

I would expect the loading of this particular workflow to be dropped because if you do not provide a replacement, you probably do not need this workflow.

Even if you wanted to preserve the workflow, you could provide a dummy image and later use the Replace Existing File function.