It seems that under certain conditions a simple "alter table" statement to rename a column can cause the database to go bad. This is what I do to reproduce the issue:

(Basically creating a table with the primary key on two columns, after that renaming one of the columns)
-------------------
create database test42;
use test42;
create table `test` ( `test_old` varchar(255) NOT NULL,
`other` varchar(255) NOT NULL,
PRIMARY KEY (`test_old`,`other`) );

alter table `test` CHANGE COLUMN `test_old` `test_new` varchar(255) NOT NULL;
select * from test;
-------------------

Up to here it works fine. Now after I restart the server (systemctl restart mariadb). I get this after rerunning the select statement.

---------------------
select * from test;
ERROR 1932 (42S02): Table 'test42.test' doesn't exist in engine
---------------------

So something went wrong with that "test" table.

BTW, the above is just a minimized test case, we orginally noticed this issue when trying to use 10.2.19 with openstack-neutron (which needs to run schemamigrations similar to the above).