we have found a bug in query API, it's rather a edge case.

Here is how to reproduce:

• Design a human task with Groups attribute filled and ExcludedOwnerId set as well

Something like this:
https://github.com/DuncanDoyle/jbpm-four-eyes-process/blob/master/four-eyes-principle/src/main/resources/four-eyes-process.bpmn2

Now try to look for this task using following API:

https://github.com/kiegroup/droolsjbpm-integration/blob/master/kie-server-parent/kie-server-remote/kie-server-client/src/main/java/org/kie/server/client/UserTaskServicesClient.java#L127

Now, we are looking for tasks only by Groups, so we don't care about User, and therefore we are setting UserId to null and only passing Groups, it looks like this:
tasks = userTaskServicesClient.findTasksAssignedAsPotentialOwner(null,Arrays.asList("SII_CHECKER"),ACTIVE_TASK_STATUSES,0,100);

So for this particular combination, when task has group as a potential owner, excluded owner is not null, and we are searching for this task with userid == null NO results are returned.

This is because of how SQL queries are written:
https://github.com/kiegroup/jbpm/blob/c398d626110d983d7af3c2e95c7e5228cbb34d32/jbpm-human-task/jbpm-human-task-jpa/src/main/resources/META-INF/Taskorm.xml#L205

that particular line will be translated to this:

and null not in
(
select exclOwner.entity_id
from
PeopleAssignments_ExclOwners exlOwner
where
task.id = exlOwner.task_id
)

and null not in
(
"someExcludedOwner"
)

which is false, because of SQL three valued logic -> this is causing no results.

If I rewrite it to this:

and '' not in
(
select exclOwner.entity_id
from
PeopleAssignments_ExclOwners exlOwner
where
task.id = exlOwner.task_id
)

i.e. I replaced null with empty string, it is evaluated as true, and it does return results - this is one of the possible solution engineering could try..

[NOTE ENG]
affects as well
TasksAssignedAsPotentialOwnerWithGroups
TasksAssignedAsPotentialOwnerByStatusWithGroups
TasksAssignedAsPotentialOwnerStatusByExpirationDate
TasksAssignedAsPotentialOwnerStatusByExpirationDateOptional
SubTasksAssignedAsPotentialOwner
QuickTasksAssignedAsPotentialOwnerWithGroupsByStatus
QuickTasksAssignedAsPotentialOwnerStatusByExpirationDateOptional
QuickTasksAssignedAsPotentialOwnerStatusByExpirationDate
NewTasksAssignedAsPotentialOwner
NewTasksAssignedAsPotentialOwnerByExpirationDateOptional
NewTasksAssignedAsPotentialOwnerByExpirationDate