Updating task and resource allocation in P6 16.1

Hello Planners,

Let's talk about Activity A, "Task dependant", on which Resource A is being allocated; duration is 5 days, budgeted units are 40h.

Project is set to recalculate allocation actual units and cost when actual duration changes.

I am updating the activity by %Duration.

When Task Duration is "Fixed duration and units" or "Fixed duration and units/time", updating the activity to 50% will set the actual and remaining units to 16.

When Task Duration is set to "Fixed units" or "Fixed Units/time", updating the activity to 50% will set the  remaining units to 16, but let the actual units to 0.

Can somebody explain this behavior? Does P6 have somewhere a new functionality that I have not discovered yet?

Thank you,

Alexandre