This is because you choose a wrong collation id.
Your collation id for utf8mb4_myown_ci is 213, which conflicts with utf8_croatian_ci, which is a utf8 collation. All your input is checked by utf8_croation_ci.
Please refer to:
https://dev.mysql.com/doc/refman/8.0/en/adding-collation-choosing-id.html to choose a good id, then it will work. (I checked this with 501).
mysql> select * from information_schema.collations order by id;
......
| utf8mb4_myown_ci | utf8mb4 | 501 | | | 8 | PAD SPACE |
+----------------------------+--------------------+-----+------------+-------------+---------+---------------+
271 rows in set (0.03 sec)
mysql> use test;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> create table t1(a varchar(10) collate utf8mb4_myown_ci);
Query OK, 0 rows affected (0.57 sec)
mysql> insert into t1 values(_utf32 0x0001f37a);
Query OK, 1 row affected (0.14 sec)