'Contains Words' has a data sensitive bug that causes it to behave seemingly flaky.
Details are findable on the forum if the forum search ever gets fixed. I remember it by the keyword: "Big Mallard Ducks". Try searching for a photo with this keyword using 'Contains Words' and the various contained words and you'll see the problem - somebody already identified detailed cause - see somewhere on the Photoshop Lightroom forum.
I never use keywords with spaces anymore, so this bug means nothing to me personally.
This one falls in the category of "should be fixed next rev" in my opinion - it has persisted for a long time now.