Can't set task duration to Zero Days

In both my original template and in my subsequent projects, there are some tasks where the system will not let me reset the duration to Zero days. These tasks are marked "As Soon As Possible", with no planned hours and set to Simple calculation.

Additionally, there is another task with the exact same predecessor, literally everything the same, that the system WILL allow me to set to Zero day duration.

I'm not sure why it keeps resetting it to 1 after I've set it to Zero.

I appreciate your help, as this is becoming a pain point!