"Configuration Item" lookup field in Problem record does not list the entire Configuration Item List as compared to the "Configuration Item" lookup field on Incident record.
Any supported release.
The "Configuration Item" reference field on task table had dictionary override setup for Problem table to have additional reference qualifier and hence due to this additional reference qualifier, the cmdb_ci field on problem form fires below query,
SELECT ... FROM cmdb cmdb0 WHERE cmdb0.`sys_class_path` LIKE '/!!%' AND cmdb0.`operational_status` NOT IN (6) /*...*/
But the cmdb_ci field lookup on incident form fires below query as there is no reference qualifier set,
SELECT ... FROM cmdb cmdb0 WHERE cmdb0.`sys_class_path` LIKE '/!!%' /*...*/
This causes the difference between the "Configuration Item" lookup on Incident and Problem record.
Review the dictionary override setup on "Configuration Item" field for "Problem" table and modify it as per the business requirement.