Execute the following SQL2 query (eg, in crx/de's query tool)
SELECT * FROM [nt:unstructured] AS c WHERE (CONTAINS(c.[jcr:title], 'a b') AND ISDESCENDANTNODE(c, '/content'))
(note there are 2 spaces between "a" and "b")
Result: java.lang.IllegalArgumentException: Invalid expression: 'a b'
If there is only 1 space between a and b, there is no error.
Per jsr-283, fulltext expressions should be able to have strings of whitespace.