This is an expected behavior and it is related to the connection pooling. After you have changed the authentication parameters, they will be applied for new connections only, while already opened sessions do not need to re-authenticate.
To avoid this situation, you can disable connection pooling(Pooling=False in the connection string) or clean pool(please refer here
dotConnect for MySQL (LinqConnect) uses connection pooling. If Pooling=true (the default value), a connection is not deleted after closing it, it is placed to the pool instead. When a new connection with the same connection string is opened, it is taken from the pool (if there are free connections) instead of creating a new one. This provides significant performance improvements.
In case Pooling=false, a connection will be deleted from memory and free the session. However this may lead to performance loss.