bug in 8.1.4. Concrete groups?

[KunihikoK3]

def myGroup := PresentedGroup {.of () ^ 3} deriving Group

gives the error

invalid dotted identifier notation, expected type is not of the form (... → C ...) where C is a constant ?m.50

also see live.lean

def myGroup := PresentedGroup {(FreeGroup.of () : FreeGroup Unit) ^ 3} deriving Group

solves the problem for me