Currently (since forever), G1 considers garbage collections that experience an evacuation failure to not count in the cpu usage calculations.

(Afair this has been introduced because the previously extremely slow evacuation failures would impact cpu usage based calculations too much even if very infrequent. For many releases evacuation failure handling is at least parallel and in the same order of magnitude wrt to performance).

This can cause significant distortions in GC behavior.

In the worst case, if for some reason all garbage collections experience evacuation failure, gc cpu usage (at least for the young gen evacuation pauses) is considered as (close to) zero, with repercussions on heap sizing.