Unfortunately, AD does not really apply schematic primitives as metric (the reasons are historical).
I can get the 'apply' button to activate if I select (or de-select - weird) 'Permanent'
But when I re-open the settings, it has reverted.
I believe this is due to the schematic library editor only working consistently in imperial units.
The grid and schematic measurement units do apparently save as metric but opening a new schematic from a template with mm defined as well as the global settings at mm: My default template below.
When I actually instantiate it I get this:
In mils, as you can see.
I can change this by using 'View' -> 'Toggle units'
But this needs to be applied on a per schematic page basis. This is a sticky setting and remains in this setting during document open / close and an AD restart.
The issue I get, though, is that because the grid is metric a lot of 'off-grid' warnings appear.
I usually just leave the schematic in imperial mode to avoid the hassle. The PCB editor is much better in this respect, but is based on a per PCB basis.
If you set the default template to metric (my default below)
Then adding a PCB will bring it in as metric. As this is really the one place I need to ensure metric, I just live with the schematic issues.