Phil,
The issue comes from using "R" instead of "r" when inputting rests. The
"R"
syntax for rests is used to indicate that an entire measure is rested,
which
centers a "whole" rest across the measure, regardless of actual duration.
It
also assumes that /there are measures/ to break up the time. The "r"
syntax
for rests doesn't care where it is, only that it takes up some amount of
time. Thus, since Mensural notation doesn't use measures, changing
"R\breve"
to "r\breve" should fix the problem for you (it worked for me).
-Abraham