Hi Div,
This used to be the default behaviour of SAP Business Workflow as well - i.e. that once one person claimed (reserved) the task, others would not see it until it was replaced by the task owner.
This behaviour has recently been changed as per SAP Note 1676067 – Automatic replacement of work items - so I think you would have a reasonable case to ask for equivalent functionality in BPM, i.e. the option to configure that if a user exits out of task without completing it that it should be unclaimed.
So I would recommend that you put your idea in the Idea Place here https://cw.sdn.sap.com/cw/community/ideas/sap_netweaver/sap_netweaver_process_orchestration_business_process_management so that it is considered for the roadmap.
In the meantime what you could consider is using the boundary events of the task and/or the API to trigger return of the task to all participants. Of course what you can do will depend on your release and support pack ... which you didn't mention? For future posts, it's always a good idea to tell us what release and support pack you are on as available features and functionality can differ greatly even between support packs.
Regards,
Jocelyn