Tasks not displaying correct tasknumber

Dear all,

Lately we are seeing some tasks that do not follow the normal task numbering. Instead of IM-999999 they have a format like 99999 and thus they appear on the bottom of the task list (and under the radar...). If you open the task you can see the correct tasknumber, so the task has a correct number.

Attached you can see an example.

How can we avoid this kind of numbering?

Thanks for your help.

PS. We run SD 7.1

Kind Regards

Koen van Carglass