Editing new custom master page based on "First"

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?

There is a thread about this where the details are explained.
The thing is that in a master page based on another one edits made in shared text frames will be reflected in both pages.
If you need individual text frames you must create these frames from scratch on both master page.

Edit:
Here is the thread:
https://www.steinberg.net/forums/viewtopic.php?f=246&t=146760&p=789203&hilit=master+page#p789203

Thanks teacue, very helpful.