This is something that really should be clearly stated in the install instructions, and it may differ per phone model;
I guess that at least for Fairphone 3 and on, stock ROM upgrade is not required, as long as /e/ have the relevant version of firmware;
“/e/ gets the relevant firmware parts from Fairphone and includes them.”
from Failed update, why (q0.17, FP3) - #4 by AnotherElk