I’ve just learned a lot from my Twitter followers and finally solved the mystery a bit. jlreq (and JIS X 4051) classifies “%” as a “postfixed abbreviation (cl-13)” rather than a Latin character, which doesn’t seem to require quarter-em spacing (according to Table 1 of jlreq). Unless it is a bug in the specification, there needs no automatic space between “20%” and “です”. Please ignore my previous statements. My apologies. 
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