Multi-Instance parallel user task and business key constraint

I have a simple use case of using a multi-intance user task, marked as parallel.
I’m usinc camunda 7.25 community edition, with a MSSQL database.

If I apply the unique business key constraint (Database Configuration |, engine fails to launch the parallel activity.

Every time when the flow enters the multi-instance activity, there are two new entries in the ACT_RU_EXECUTION table. One for the multiInstanceBody, which has null value for business key. And the second entry is the user task and has the proper business key.

Could you please advise wheather this is a normal behaviour.
Thanks in advance