if you're interested, here is the response of atlassian support
I'm afraid that this is happening because you do have your SLA fields (SLA Prise en charge and SLA Résolution) on the edit issue screen of your project.
Please note that SLA fields are internally managed by JIRA Service Desk, users can't manipulate them. When we add an SLA field to an edit screen it won't be displayed, and when the user submits the update JIRA subscribes the SLA field with null value.
Please try removing those fields from your edit issue screens and let us know how it goes.
I tried by removing SLA fields on the screen, it's OK.
...be more productive while being fun to use at the same time. For some, getting started can be a bit intimidating. This is especially true if Jira Service Desk is your first exposure to Atlassian...
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