When running exclusive compute unit jobs (with the resource requirement cu[excl]), the advance reservation can affect hosts outside the advance reservation but in the same compute unit as follows:
An exclusive compute unit job dispatched to a host inside the advance reservation will lock the entire compute unit, including any hosts outside the advance reservation.
An exclusive compute unit job dispatched to a host outside the advance reservation will lock the entire compute unit, including any hosts inside the advance reservation.
Ideally all hosts belonging to a compute unit should be inside or outside of an advance reservation.