Additionally

as regards 1): I am not a programmer myself and don't know if it makes sense to support JSON string > TEXT (64K) at all. But there is at least a theoretical chance of truncation when moving data from MySQL to MariaDB.

as regards 2) I think it makes perfectly sense to enforce utf8mb4 for JSON becuase of emojis, that are mapped outside the BMP in Unicode and thus not even supproted with utf8.  Emojis are getting increasingly popular with web applications and can be expected to replace smileys based on mapping of ASCII character sequences (such as "." + "-" + ")").


-- Peter

On Thu, Jul 13, 2017 at 10:50 AM, Peter Laursen <peter_laursen@webyog.com> wrote:
 -- MariaDB 10.2.7
CREATE TABLE `jsontst`( `id` INT NOT NULL AUTO_INCREMENT, `str` JSON, PRIMARY KEY (`id`) ) ENGINE=Aria CHARSET=latin1;
SHOW CREATE TABLE `jsontst`;
/* returns 
CREATE TABLE `jsontst2` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `str` text DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=Aria DEFAULT CHARSET=latin1 PAGE_CHECKSUM=1
*/


readings:

As you see except for JSON in MariaDB just being a synonym of TEXT there are two more significant differences
1) length: in MySQL JSON has the lenght of a LONGTEXT - in MariaDB it is a (plain) TEXT
2) encoding: in MySQL the utf8mb4 character set is enforced for JSON data - in MariaDB you may specify any character set.

I think in particular 2nd point could cause problems (unicode garbling) when loading a dump with JSON data from MySQL. Basically because latin1 is still default character set in MariaDB.

Are you aware of the 2 differences? I think the major reason for introducing JSON in MariaDB is compability concerns with MySQL.  But the 2 points here are IMO incompabilites that could have been avoided?