SSGI doesn't understand dynamic resolution scaling, and the history buffer gets invalidated on resolution change.
This makes the dynamic aspect of the DRS feature unusable with SSGI.
This bug is a companion bug to "Dynamic Resolution Scaling: blinking raytraced reflection denoising" https://fogbugz.unity3d.com/f/cases/1418319/ . Both issues are visible in the below clip and reproducible in the attached project.