Flying B738.
Volume 2, Pg. 4.10.9
VNAV mode is terminated by any one of the following :
Crosstrack deviation exceeds twice the RNP value during PTH descent for
an active leg with a database vertical angle and LNAV not engaged.
( There are other reasons for termination of VNAV mode but this was the one
relating to the question ).
Seen it happen this morning.