En la vista antigua si me permite cambiar el responsable, he realizado la prueba sobre el mismo ticket, sin embargo, en la nueva vista no me permite editar el responsable.
He revisado los permisos y pertenezco a las grupos que si se les permite cambiar responsable
As we will need to further investigate this issue by accessing your instance, I've opened a ticket on your behalf, so we can check the permissions and confirm what is causing this issue.
I'll be watching the ticket and once it's resolved, I'll post here as well.
I already added a summary on the ticket to get easier to investigate.
You will receive a confirmation email and we will contact you soon.
If you click on Responsable, can you edit the assignee?
If not, please go to the Configuración de proyecto > Permisos and check Usuario asignable and Asignar incidencias. It is usually set to Cualquier usuario conectado, so anyone can assign a ticket to someone and also anyone can be the assignee of a ticket.
I've tested on my test instance and it works on any screen when using the new issue view.
Please, give a try and let us know how it goes.
I've just tested on my test instance and it's possible to edit the assignee:
Just to make sure that we are on the same page, can you please send us a screenshot of where you are trying to change the assignee? Perhaps it's a specific page and we must check if it's a bug.