I think that both are correct.
And I can justify this claim also: Just consider the two body formulas (a | []y) & [](a | y) and (a | []y) & (!a | [](a | y)) and make a case distinction to prove (a | []y) & [](a | y) = (a | []y) & (!a | [](a | y)):
- a=0: then, we get ([]y) & [](y) = ([]y) which is true
- a=1: then, we get (1) & [](1) = (1) & ([](1)) which is also true
So, the fixpoints are the same.