Master pages' Room for Side Heads not being honoured

[Mark Lehmann]

I have a book file containing all the usual suspects: a front matter 
file, TOC, LOF and LOT files, assorted chapter and appendix files, an IX 
(index) file and an end file. [FM10, Win7 SP3]
I've created two master pages, Right_Index and Left_Index, and applied 
them to the appropriate pages in the IX file. The two IX master pages 
differ from the main Left and Right master pages in that they have Room 
for Side Heads un-ticked and Columns = 2, so that the IX pages are all 
2-column with no room for sideheads (duh).
I've propagated these two master pages to all other files in the book, 
and the IX pages look correct.
So far, so good.

But . . .
After I import Page Layouts into the IX file from another file in the 
book, the IX pages all have Room for Side Heads ticked!
Checking the IX file shows that Right_Index and Left_Index master pages 
are correctly applied, and they correctly show Room for Side Heads = off 
. . . but the document pages now all have Room for Side Heads = on. (And 
if I open master pages then go back to document pages, I'm NOT warned 
about overrides.)
But behold, there's more: if I manually set Room for Side Heads = off 
for the main text frame on the first page of the IX file, ALL pages in 
the file come good.

Any ideas what's going on?
How can I fix things so that I don't have to remember to do this manual 
tweak?

Cheers,
Mark Lehmann