range chosen for update while ref used for select on multicolumn index with length

slow query log:

# Thread_id: 161  Schema: hotspot  QC_hit: No

# Query_time: 17.934323  Lock_time: 0.000119  Rows_sent: 0  Rows_examined: 4

#

# explain: id   select_type     table   type    possible_keys   key     key_len ref     rows    Extra

# explain: 1    SIMPLE  radacct range   AcctSessionId,UserName_AcctSessionId,UserStop   UserName_AcctSessionId  52      NULL    1       Using where; Using buffer

MariaDB [hotspot]> explain UPDATE radacct           SET ..       WHERE AcctSessionId = '80504297'           AND UserName = 'tZZZZZZ'\G

*************************** 1. row ***************************

           id: 1

  select_type: SIMPLE

        table: radacct

         type: range

possible_keys: AcctSessionId,UserName_AcctSessionId,UserStop

          key: UserName_AcctSessionId

      key_len: 52

          ref: NULL

         rows: 1

        Extra: Using where; Using buffer

MariaDB [hotspot]>explain select * from radacct     WHERE AcctSessionId = '80504297'           AND UserName = 'tZZZZZZ'\G

*************************** 1. row ***************************

           id: 1

  select_type: SIMPLE

        table: radacct

         type: ref

possible_keys: AcctSessionId,UserName_AcctSessionId,UserStop

          key: UserName_AcctSessionId

      key_len: 52

          ref: const,const

         rows: 1

        Extra: Using where

1 row in set (0.00 sec)

CREATE TABLE `radacct` (

  `RadAcctId` bigint(21) NOT NULL AUTO_INCREMENT,

  `AcctSessionId` varchar(32) NOT NULL DEFAULT '',

  `AcctUniqueId` varchar(32) NOT NULL DEFAULT '',

  `UserName` varchar(64) NOT NULL DEFAULT '',

....

  PRIMARY KEY (`RadAcctId`),

  KEY `UserName_AcctSessionId` (`UserName`(8),`AcctSessionId`(8)),

...

) ENGINE=InnoDB AUTO_INCREMENT=141873298 DEFAULT CHARSET=utf8