Verified with PS 5.6.25. Also checked with MySQL Community 5.6.25 but its not reproducible there. 



nilnandan@desktop:~$ mysql -uroot -p
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 40
Server version: 5.6.25-73.1-log Percona Server (GPL), Release 73.1, Revision 07b797f

Copyright (c) 2009-2015 Percona LLC and/or its affiliates
Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql>  grant select on test.* to 'testuser'@'%' identified by 'test';
Query OK, 0 rows affected (0.00 sec)

mysql> grant select on test.* to 'testuser'@'10.10.10.%' identified by 'test';
Query OK, 0 rows affected (0.00 sec)

mysql> show grants for 'testuser'@'10.10.10.%';
+------------------------------------------------------------------------------------------------------------------+
| Grants for testuser@10.10.10.%                                                                                   |
+------------------------------------------------------------------------------------------------------------------+
| GRANT USAGE ON *.* TO 'testuser'@'10.10.10.%' IDENTIFIED BY PASSWORD '*94BDCEBE19083CE2A1F959FD02F964C7AF4CFC29' |
| GRANT SELECT ON `test`.* TO 'testuser'@'10.10.10.%'                                                              |
| GRANT SELECT ON `test`.* TO 'testuser'@'%'                                                                       |
+------------------------------------------------------------------------------------------------------------------+
3 rows in set (0.00 sec)

mysql> 


With community server, 

nilnandan.joshi@bm-support01:~$ mysql -uroot -p --socket=/tmp/mysql_sandbox27002.sock
Warning: Using unique option prefix pass instead of password is deprecated and will be removed in a future release. Please use the full name instead.
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 4
Server version: 5.6.25-log MySQL Community Server (GPL)

Copyright (c) 2009-2015 Percona LLC and/or its affiliates
Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> grant select on test.* to 'testuser'@'%' identified by 'test';
Query OK, 0 rows affected (0.09 sec)

mysql> grant select on test.* to 'testuser'@'10.10.10.%' identified by 'test';
Query OK, 0 rows affected (0.00 sec)

mysql> show grants for 'testuser'@'10.10.10.%';
+------------------------------------------------------------------------------------------------------------------+
| Grants for testuser@10.10.10.%                                                                                   |
+------------------------------------------------------------------------------------------------------------------+
| GRANT USAGE ON *.* TO 'testuser'@'10.10.10.%' IDENTIFIED BY PASSWORD '*94BDCEBE19083CE2A1F959FD02F964C7AF4CFC29' |
| GRANT SELECT ON `test`.* TO 'testuser'@'10.10.10.%'                                                              |
+------------------------------------------------------------------------------------------------------------------+
2 rows in set (0.00 sec)

mysql>