ER_NOT_SUPPORTED_AUTH_MODE with mysql_native_password

I have a user set up with mysql_native_password authentication, verified here:

mysql> select user, plugin from mysql.user where user="testuser";
+----------+-----------------------+
| user     | plugin                |
+----------+-----------------------+
| testuser | mysql_native_password |
+----------+-----------------------+

I also made sure to flush privileges to ensure everything was updated on MySQL's side, but when I try to log in, it's still giving me an ER_NOT_SUPPORTED_AUTH_MODE.

MySQL v8.0.19 Library version 2.18.1