Changing "LONGTEXT" column's collation (using ALTER TABLE ... CHANGE) truncates data after 65535 chars

Changing the collation of a LONGTEXT column (using ALTER TABLE ... CHANGE) truncates the data contained in it to exactly 65535 characters long (the max length of a TEXT type column?).

I've confirmed/reproduced this with MariaDB 5.5.64 and 10.0.38. MySQL 5.6.13 doesn't experience this behaviour.

The problematic query is in the form of:

ALTER TABLE <table_name> CHANGE <column_name> <column_name> LONGTEXT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL

The following work fine/as expected:

ALTER TABLE <table_name> MODIFY <column_name> LONGTEXT CHARACTER SET utf8mb4

and:

ALTER TABLE <table_name> CONVERT TO CHARACTER SET utf8mb4

(The specific collation or the type of data contained in the LONGTEXT column doesn't seem to matter; I'm able to reproduce it every time. Also both InnoDB and MyISAM storage engines experience the same behaviour.)

Side notes

I originally posted this issue on PhpMyAdmin's github page and got directed here: https://github.com/phpmyadmin/phpmyadmin/issues/15670

I hope I've included enough information.