Field-Level Security for Permission Sets during Field Creation (Beta) bug detected ?

I found some strange behavior on the new Field Level Security feature for permission sets when creating a field (beta).

When I create a new field on Activity and assign it to a permission set, some other permission sets lose their settings on the object bound to that new field.

Steps to reproduce : 

1. Create a permissions set "Read Only Event" and add read only right on all event fields
2. Create a new field on Activity and assign it to another permission set (no assignation for the perm set "Read Only Event")
3. Go back to the permissions set "Read Only Event" and resfresh the page
4. you will see event fields permissions unchecked