sprint kills other sprints

Hi, I give you our scenario:
I created (not started) a sprint where an issue "a" is planned (assigned to me). This issue is linked to an issue "b" which is assigned to another one and is planned in another sprint. The other sprint (with issue "b" in it) was started. When I start my sprint (with issue "a" in it) the sprint with issue "b" is automatically completed and all issues fall back to backlog.
Is this a known bug or are we doing something wrong? But even when we doing something wrong, there should be a message or any warning before deleting another sprint.
I don' know if it is relevant but we are using a custom field named "target group" where we identify the team which handles the issue.
Greets Riccardo.