Dropdown Slicer selects all by default

I'm getting different behaviour from two slicers and can't figure out why, and I need them to behave the same.

Both are dropdowns and have identical slicer settings.

On the first slicer "Owner", the list shows all values are un-ticked, allowing the user to search or select multiple values - perfect 🙂

But the second slicer "System Level 1" shows all values ticked, forcing a user fiddle with un-ticking values or un-ticking 'select all' and then tick their selection - this is aggrevating and confusing for users.

What am I missing? There must be some setting for this, I need slicer dropdowns to start with all values unselected.