Why did the data type of my DB change?
The issue related to JSONB columns is a known limitation of DMS. In particular, if you're using DMS to migrate from your EC2 instance to RDS Postgres, it will convert the type to varchar(765) and truncate any JSONB column longer than 756 chars.

https://forums.aws.amazon.com/message.jspa?messageID=735961

We heard during AWS Summit - San Francisco that the DMS team is planning a fix for this issue. For an update, please feel free to comment on the message itself.