I've seen seismic anchor motion handled both ways - as strain included in the expansion stress range and as loads contributing to occasional stress.

B31E (the general B31 document on seismic evaluation of piping systems) kind of straddles the fence. Bending moments used to evaluate (sustained plus occasional) stress include "relative anchor motion". But there is a second stress check - the "resultant force (tension plus shear) due
to seismic anchor motion" divided by pipe cross sectional area must be less than or equal to pipe material yield strength.