v$asm_disk preferred_read is sometimes "U", why ?

Greetings community,

using ASM 12.1.0.2 on AIX 6.1

We have some diskgroups with redundancy normal, using the asm_preferred_read_failure_groups initialization parameter : the preferred_read column of v$asm_diskgroup is set to Y or N for each of their disks. This is the expected behaviour.

Some other diskgroups have external redundancy : the preferred_read column for their disks is set sometimes to null and sometimes to "U"

Example : here is the count of disks per diskgroup + failgroup + preferred_read column

SQL> set pagesize 2000

set linesize 200

col path format a30

select g.name, g.type, d.preferred_read, count(*) DISK_COUNT