I’m trying to create a new master page for my Part Master Page set - it’s very similar to the normal “First” page, but it’s slightly different.
I created a new master page, based on First, and selected the type as Custom.
The new master page is created, and it starts off looking like the First master page, which I expect. But when I edit that new master page, it copies those changes to my existing First master page!
I thought that I could use the design of “First” as a starting point, and edit it without affecting the actual First master page, since I set it as “Custom.” Am I misunderstanding?