Det verkar vara fixat i 10.1.22.
Server version: 10.1.22-MariaDB-1~xenial mariadb.org binary distribution
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Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
MariaDB [(none)]> select 'a%b' like '%\%\%';
+--------------------+
| 'a%b' like '%\%\%' |
+--------------------+
| 0 |
+--------------------+
1 row in set (0.00 sec)