[circonian]

This probably isn't very helpful, but here is a snippet I found in the lua files. It looks like they call a delay when a unit is created (or destroyed) and then call self:BuildMasterList() to rebuild all of the group data from scratch. So whatever is causing it, this is probably a known issue. They do the same when a unit is destroyed.

ZO_GroupList_Manager.lua

function ZO_GroupList_Manager:RegisterForEvents()
    --  During group invitation, we can receive a lot of event spam at once on a single invite when the
     -- involved players are at the same location. Add a delay so we only refresh once in cases like this.
    local function DelayedRefreshData()
        self.delayedRebuildCounter = self.delayedRebuildCounter - 1
        if self.delayedRebuildCounter == 0 then
            self:RefreshData()
        end
    end
 
    local function RegisterDelayedRefresh()
        self.delayedRebuildCounter = self.delayedRebuildCounter + 1
        zo_callLater(DelayedRefreshData, 50)
    end
 
  ...
end
 
EVENT_MANAGER:RegisterForEvent("ZO_GroupList_OnUnitCreated", EVENT_UNIT_CREATED, RegisterDelayedRefreshOnUnitEvent)
EVENT_MANAGER:RegisterForEvent("ZO_GroupList_OnUnitDestroyed", EVENT_UNIT_DESTROYED, RegisterDelayedRefreshOnUnitEvent)