That was mentioned multiple times on forums:
FS Acars is not exacly working as intended. Use at own risk.
I would use x-acars in Your place, it works with fs same as with x-plane. It has some minor flaws with live server connection (and managing flight distance log) but works for most time, even offline.
I mean, that IF xacars loose connection to live server it stops calculating flown distance, so You will have full pirep but with incorrect distance. As far as i remember currentlyflight distance from xacars is calculated on server side, because calculated in xacars is not sent in pirep.