For what it's worth, I ran this in grid mode in SSMS:
declare @json table (j nvarchar(max));
insert into @json SELECT *FROM (SELECT TOP 200 * FROM Customers FOR JSON AUTO )a(j)
select len(j), * from @json
I copied the contents in the second grid cell to the query window and went to the end of the line. It did indeed seem like the end of a JSON document. I also noticed that it said "Col 54546" in the status bar, and that agreed with the output from the len function.
It appears that Melissa is not updated on SSMS. Since some release, you can set the maximum length for a grid cell to 2 million, 2000000.
However, how much I hate to say it, the best option when you work with JSON is Azure Data Studio. SSMS is crippled when it comes to JSON.