slow_query_log_use_global_control=all doesn't mean all

Setting slow_query_log_use_global_control=all does not set all possible values (especially long_query_time):

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(root@localhost) [(none)]> show global variables like 'slow_query_log_use_global_control';
+-----------------------------------+-------+
| Variable_name | Value |
+-----------------------------------+-------+
| slow_query_log_use_global_control | |
+-----------------------------------+-------+
1 row in set (0.00 sec)

(root@localhost) [(none)]> set global slow_query_log_use_global_control='all';
Query OK, 0 rows affected (0.00 sec)

(root@localhost) [(none)]> show global variables like 'slow_query_log_use_global_control';
+-----------------------------------+--------------------------------------------------------+
| Variable_name | Value |
+-----------------------------------+--------------------------------------------------------+
| slow_query_log_use_global_control | log_slow_filter,log_slow_rate_limit,log_slow_verbosity |
+-----------------------------------+--------------------------------------------------------+
1 row in set (0.00 sec)

(root@localhost) [(none)]> set global slow_query_log_use_global_control="log_slow_filter,log_slow_rate_limit,log_slow_verbosity,long_query_time";
Query OK, 0 rows affected (0.00 sec)

(root@localhost) [(none)]> show global variables like 'slow_query_log_use_global_control';
+-----------------------------------+------------------------------------------------------------------------+
| Variable_name | Value |
+-----------------------------------+------------------------------------------------------------------------+
| slow_query_log_use_global_control | log_slow_filter,log_slow_rate_limit,log_slow_verbosity,long_query_time |
+-----------------------------------+------------------------------------------------------------------------+
1 row in set (0.00 sec)

This is a pain -- especially when trying to force already-connected clients to use the global long_query_time.