strange behaviour with dotted note before gregorian music

[Ali Cuota]

Hello again,

version "2.18.0"
I have a western-notation piece that finishes with a dotted-note a4.
(very last note)
then a gregorian piece.
both comes in a book.ly with include "..."
Compiling the sole gregorian is ok
Compiling the book makes the gregorian have dotted (augmentum) at
every notes except \pes
This is note so terriffic since putting a s8*1/1000 at the western
piece or at the beginning of the gregorian piece solves the problem,
but I would say this is a bug...

Franck