Delete Process Instance Async Returning Success But Process Instances Remain

Hi all

We have an instance of the process engine running that has several thousand process instances blocked on an external task. They were created as part of security testing and we need to remove them.

I have tried to delete all these process instances using the async delete rest endpoint ( Delete Async (POST) | docs.camunda.org)

I am getting the expected success response and in Cockpit I can see the batch job that is created however the job never completes (and none of the instances are deleted)

If I delete an instance using the DELETE rest endpoint for a specific instance ID it works correctly so I know I could write a script to loop but I would like to understand why the async delete is not working like I expect.

Can anyone point me in the right direction?

It seems similar to this topic however that never seemed to be satisfactorarily resolved: Job executor not picking up job - Process Engine - Camunda Platform Forum

For way of context this Camunda engine is being run inside a docker container and has been redeployed several times since the process instances were created (because of introducing basic auth)

I am new to Camunda and have inherited this system so any assistance greatly appreciated.

Regards
Mark