Dunno how long it's been like this, and if it's a bug or a feature:
When I lock the zoom level and zoom out, I cannot zoom out below 100%. Some while ago, it was possible to zoom out to <100%, so you could see a bigger area of a more detailed high resolution zoom level. Granted, at some point the repainting of the tiles got reaaaally slow, so I can imagine that might have been a reason to disallow it. However, I think it would be unproblematic and still usable if it's at least allowed to zoom out to 25%.