The FO included shows two page-sequences using a conditional-page-master-reference.

Note that the "last" simple-page-master (SPM) as a smaller region body than the
"first" and "rest" SPM.

1) The first page-sequence flows the text in the "first" simple-page-master
(SPM). The end flow doesn't fit in the "last" SPM, so it flow in a "rest" SPM
and then append an empty "last" SPM. This is correct, the last page is a "last"
SPM layout.
This gives 3 pages: FIRST, REST, LAST

2) The second page-sequence is identical to the first, apart from the
force-page-count="even" attribute. In this case. We get 4 pages as expected by
the force-page-count to even.
But we get FIRST, REST, LAST, REST, instead of FIRST, REST, REST, LAST.

Guillaume.