Neil
Have you ever been called out to treat a spillage/stain on a new carpet? Did the treated area turn lighter in shade than the rest? It's happened to me with just a cold, freshwater rinse from my spotting machine and I've ended up cleaning a whole carpet (and charging for it).
New carpets are rarely clean. Sometimes, the yarn has been treated with a lubricant that will walk off in time, but this can retain soil too. This lubricant can help reduce friction from the yarn when passing through the looms. One of my local CD franchisees claims that a CD cleaned carpet can stay clean longer than a new carpet, and that this has been tested by CAMRASO. They're not telling lies, but, IMO, they're only drawing a rough sketch, not painting the full picture.
It may be this lubricant that is resisting yarn penetration from cold liquids, with the hot spillages suspending the lubricant immediately. Try cleaning the samples first, finish with an acid rinse if appropriate to your protector, allow to dry and then do your testing.
Safe and happy cleaning
The Ken