No, it is right behavior

Planning activity can be link to several tickets.
Any ticket can be closed without having to close the activity.
The activity cannot be closed unless all tickets linked are closed.

I think this is wrong! Ticket is primary and the activity is the secondary, so Ticket should not be closed until activity is closed!

No, it is not designed this way.
Planning activity is the global point to gather work for several tickets, and be able to plan some activity for tickets that are not known yet, so it comes firts.
It is for instance used to plan some "maintenance" activity. You know you'll have to work on tickets, but don't know the tickets yet.

I think you mismatch with an activity that you could link to ticket, for instance copying ticket into activity, the ticket being the "origin" of the ticket.

Regards.

Tickets suppose to come, for example, from customers, so you need to create activity to deal with it not the opposite way...

Hi Evgueni,

we use it right in the way as described by Babynus. We collecting and linking Tickets to one Activity. In some caes it might be the case, a Tickets is rejected, but the remaining Tickets and work has to be performed with the Activity. But an Activity should have closed or done all linked Tickets, before Activity is closed.

So from my pov it's correct as it is.

Regards,

Klaus