[MSaint]

Ok, here is the fixed up version of CreateHeader():

function GridLayout:CreateHeader(isPetGroup)
   NUM_HEADERS = NUM_HEADERS + 1

   local template = isPetGroup and "SecureGroupPetHeaderTemplate" or "SecureGroupHeaderTemplate"
   if ClickCastHeader then
      template = template .. ", ClickCastUnitTemplate"
   end

   local header = CreateFrame("Frame", "GridLayoutHeader" .. NUM_HEADERS, GridLayoutFrame, template)

   for k, v in pairs(self.prototype) do
      header[k] = v
   end

   header:SetAttribute("template", "SecureUnitButtonTemplate")
   
   -- Fix for bug on the Blizz end when using SecureActionButtonTemplate with SecureGroupPetHeaderTemplate
   if isPetGroup then
      header:SetAttribute("useOwnerUnit", true)
      header:SetAttribute("unitsuffix", "pet")
   end
   ...

And then you have to catch any cases where a layout tries to set the useOwnerUnit attribute in GridLayout:LoadLayout():

      ...
      -- apply defaults
      if defaults then
         for attr, value in pairs(defaults) do
            if attr == "unitsPerColumn" then
               layoutGroup:SetAttribute("unitsPerColumn", value)
               layoutGroup:SetAttribute("columnSpacing", p.Padding)
               layoutGroup:SetAttribute("columnAnchorPoint", getColumnAnchorPoint(p.groupAnchor, p.horizontal))
            elseif attr == "useOwnerUnit" then -- related to fix for using SecureActionButtonTemplate, see GridLayout:CreateHeader() 
               if value == true then
                  layoutGroup:SetAttribute("unitsuffix", nil)
               -- else do nothing or you will break things!
               end
            else
               layoutGroup:SetAttribute(attr, value)
            end
         end
      end

      -- apply settings
      for attr, value in pairs(l) do
         if attr == "unitsPerColumn" then
             layoutGroup:SetAttribute("unitsPerColumn", value)
             layoutGroup:SetAttribute("columnSpacing", p.Padding)
             layoutGroup:SetAttribute("columnAnchorPoint",  getColumnAnchorPoint(p.groupAnchor, p.horizontal))
         elseif attr == "useOwnerUnit" then -- related to fix for using SecureActionButtonTemplate, see GridLayout:CreateHeader() 
            if value == true then
               layoutGroup:SetAttribute("unitsuffix", nil)
            -- else do nothing or you will break things!
            end
         elseif attr ~= "isPetGroup" then
            layoutGroup:SetAttribute(attr, value)
         end
      end
      ...