I tried to reproduce this using the following procedure. After restarting MariaDB, user.frm file is lost. Is this issue already fixed?
In the other hands, mysql.spider_tables table needs to repair after restarting. This issue is fixed at MDEV-16250.

MariaDB [test]> CREATE OR REPLACE TABLE user(
-> `Host` char(60) COLLATE utf8_bin NOT NULL DEFAULT ''
-> )ENGINE = SPIDER
-> COMMENT 'host "127.0.0.1", port "3312", user "root", password "", database "mysql", table "user"';
Query OK, 0 rows affected (0.006 sec)

MariaDB [test]> show create table user\G
//*************************** 1. row ***************************
Table: user
Create Table: CREATE TABLE `user` (
`Host` char(60) CHARACTER SET utf8 COLLATE utf8_bin NOT NULL DEFAULT ''
) ENGINE=SPIDER DEFAULT CHARSET=utf8mb4 COMMENT='host "127.0.0.1", port "3312", user "root", password "", database "mysql", table "user"'
1 row in set (0.000 sec)

MariaDB [test]> CREATE OR REPLACE TABLE user
-> ENGINE = SPIDER
-> COMMENT 'host "127.0.0.1", port "3312", user "root", password "", database "mysql", table "user"';
ERROR 2013 (HY000): Lost connection to MySQL server during query
MariaDB [test]> show create table user\G
ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id: 2
Current database: test

ERROR 1146 (42S02): Table 'test.user' doesn't exist

After MDEV-11412 DROP should always work.
After MDEV-25292 CREATE OR REPLACE won't destroy the existing table if creation failed.