I should have pointed to the gerrit that's an external tracker on that. It shows the root cause is that the disk_image_dynamic table (part of the engine) isn't getting updated. It's a view of that table that's used for the stats that you see online and is used by the collector for dwh.
http://gerrit.ovirt.org/#/c/12961/It says
core: disk_image_dynamic statistics (#892596)
vdsm sends disk statistics per disk, while disk_image_dynamic table
contains disk_image ids (volumes). Hence the update on disk ids that
VdsUpdateRunTimeInfo generates, actually does nothing. A quick fix for
this problem is that VdsUpdateRunTimeInfo now updates by the AI id, so
statistics are saved.