The FSRS search introduced in 2.1.64 has a few problems that I’ve noticed.
prop:cdn:r>0.9 searches for cards that had a retention (retrievability?) greater than 90% when it was last reviewed. Instead, it should search for cards that have a current retention greater than 90%. Retention at last review is a relatively meaningless statistic. Searching for r>0.9 (with the FSRS helper addon installed) gives a more accurate result, however, r>0.9 doesn’t work in a filtered deck which is unfortunate.
The FSRS helper addon has the ability to search for r,d,s in a different way from how anki natively does it. The two versions of the search (prop:cdn:s>40 and s>40) (prop:cdn:d>5 and d>5) result in a different number of cards. prop:cdn tends to show more cards than the FSRS helper. Not sure which one is more accurate.