Then we have two formats again. Only a bit less different. There must be more possibilities to solve this for everyone.

Is there a way we can fix the order implicitly?
Or can we assure that the map is always written into Json in the correct order?
Maybe use a combination of Property and index as key?

There is not really a lot of possible or plausible implementations. But the real question is : is this worth the headeach ? Premature optimisation is the root of all evils.

As a user, I would prefer you to focus on problems that really have an impact than to whatever question of having only one serialisation format, which is really a stractch in the big picture.