Restrict access to SharePoint homepage to 'view only'

Hi. I need to adjust a group's access to a site homepage to 'can view'. (Not the whole site - just the site homepage). 

I can usually do this in Site Contents > Site Pages > find the page concerned > Manage Access - where I have always be able to change the group (which has contribute permissions to the site itself) to can view. 

Today, I find the function in here has changed. Everything is greyed out, so I can't change it. 

This is annoying because I have 2 permission levels per site, editors and contributors, and I do not want the contributors to be able to edit the site homepage. 

This is happening on multiple tenancies. I'm a site collection admin and site owner. 
If anyone can help - I'd be very grateful.