rollback segments remain PENDING OFFLINE indefinitely

Oracle 11.2.0.4

manual undo used cause of bug that gave:

ORA-1628: max # extents 32765 reached for rollback segment

----

select b.name, a.xacts, a.status

from v$rollstat a, v$rollname b

where a.usn = b.usn

order by name;

shows PENDING OFFLINE status with number of active transactions

NAME XACTS STATUS

RBS_01 2 PENDING OFFLINE

RBS_03 2 PENDING OFFLINE

RBS_04 3 PENDING OFFLINE

...

The problem is all queries to find these active transactions return no rows 

and active transactions cannot be there after reboot

no in-doubt remote transactions

forcing these segments offline in pfile has no effect 

_offline_rollback_segments= "('rbs_01', 'rbs_03', 'rbs_04')"