The release notes for 8.17 don't mention #4421 so it took me a bit longer to narrow it down ;)

Bug report

The referenced change adds a new scaleFactor within the tiles array of a IIIF info.json file. This can confuse some IIIF Viewers. I've tested against OpenLayers and OpenSeadragon.
While it's not really noticeble using OpenLayers, since it transparently falls back to the next possible tile set and only logs the problem to the console, OpenLayers isn't as forgiving by default...

You can test OpenLayers here:
https://openlayers.org/en/latest/examples/iiif.html

For the next few days one can use this example. But it will be replaced with a file regenerated by Vips 8.16.1 very soon:
https://static.projektemacher.org/backsteinexpressionismus/post/messegelaende-koeln-3/front/info.json

When opening with the OpenLayers demonstrator you can see a request to
https://static.projektemacher.org/backsteinexpressionismus/post/messegelaende-koeln-3/front/full/439,/0/default.jpg
Which hasn't been generated by VIPS. For reference, this is the generated info.json:

{
  "@context": "http://iiif.io/api/image/2/context.json",
  "@id": "https://static.projektemacher.org/backsteinexpressionismus/post/messegelaende-koeln-3/front",
  "profile": [
    "http://iiif.io/api/image/2/level0.json",
    {
      "formats": [
        "jpg"
      ],
      "qualities": [
        "default"
      ]
    }
  ],
  "protocol": "http://iiif.io/api/image",
  "tiles": [
    {
      "scaleFactors": [
        1,
        2,
        4,
        8
      ],
      "width": 512
    }
  ],
  "width": 3509,
  "height": 2506
}

The scaleFactor of 8 causes the viewer to fetch an non-exiting image.