While running the tests on https://github.com/danielcompton/data.fressian-test I found this bug:
Changing the key  to :x
or removing another kv pair
will make the roundtrip encoding of Fressian values equal.
This is because of the changes in hashing behaviour introduced in 1.6 and relates to CLJ-1372. I think there may also be some interaction with Fressian's creation of ArrayMaps for maps with less than 8 kv pairs.
One fix would be to make sure data.fressian always returns persistent Clojure data structures, so it avoids tripping over Clojure no longer having Java data structures be equal to their analogous Clojure ones.