Format Read only condition - Based on logged in user

I am attempting to lock a field down based on the logged in user (whether they are the assignee or not)

Background: the request tasks module is set to allow edits to the form (based on process designer workflow conditions) to members of the assignment group. This works fine, however other areas (like updates, status etc) can be edited without the task being assigned to the logged in user.

The assignment group members still need to be able to update the assigned.to field, but I need to lock the status field down when it isn't assigned to them. This seems like it should work, but it does not:

[assigned.to]<>$lo.user.name

[assigned.to]<>"{specific username}" works, as does [assigned.to]<>null, so I can only assume for some reason it is not evaluating the specific variable. I did also try 'operator()' but I had no success there either.

This is a 9.41 process designer hybrid system, (though the module I am working in is a PD enabled module)