I had some time so I poked around for the thumbnail generator. The function isImage is correctly identifying TIF as an image but claviska/simpleimage doesn't appear to support TIF files.

I see that in setEmbeddableType you are relying on isImage() to give files the greenlight, so any image format that isn't supported by simpleimage is waved through. On my branch I changed it to

        // Image
        $embeddable = ["gif", "jpeg", "png", "webp", "bmp", "svg"];
        if (isImage($this->full_path) && in_array($this->extension, $embeddable)) {
            $this->embeddable = true;
            $this->embeddable_type = 'image';
        }

However, this didn't stop the broken URLs from being generated and appearing, so something else must be up. It seems that sometimes we check for $file->embeddable and sometimes we don't. I tried poking at it and shattered everything so I'm gonna back off for now. I might take another look when I have some time.