Duplicating master page (that is based on another master page) is producing un-editable content

I have never seen this oddity (and very frustrating) mentioned anywhere... (maybe it's the version of Indd I'm using? 15.1.2 x64... and unfortunately, there's a legit reason we're still on this version...updating at this time isn't an option)

So, I have a facing pages, 2pg master "A-BASE Master" with pg # placeholder... see below screen capture.

Then I have a B-... master page with other content, set to Based On "A-BASE Master"  (these are used in a book, so the A-BASE Master pages are synced through whole book, but not every doc in book uses the B-... master page because different parts of the book use different elements... So B-... looks like this:

Here's the part that's maddening: If I move or duplicate the B-... master page, the resulting master page is not a direct duplicate... it get's messed up in a significant way! And you can't move/edit the screw-up in any way! See what happens to the page # text frame... It shifts over to the left! 

What!##%&@  And this is terrible in my book environment. Syncing the book in order to push these master pgs replicates the same screw-up in all the doc books... (because the syncing involves "duplicating") I also found the same thing happens if I Move the master pg from this file to another...

Has anyone else ever seen this and why would it be doing this?