This appears to be an issue in the firmware with ui autoscaling and 9-patch images.
Most channels I work with are only
ui_resolutions=fhd
In this case, 9-patch images appear to not scale correctly for 720p UI so I always have to make multiple versions of the 9-patch images, then use:
uri_resolution_autosub=$$RES$$,sd,hd,fhd