[lightspark]

Well, it had to happen because the update process happens in stages now, filtering > sorting > redrawing. Buttons get updated only at the redrawing stage, but before that we work with raw aura data. If I were you, I'd use PostUpdateButton instead of PostUpdate since it gives you both the button and the aura data, but you won't need the latter in this case. I drycoded it, but this should do the trick.

local BOLSTER_ID = 209859
 
local function PreUpdate(element)
    element.bolsterStacks = 0
    element.bolsterInstanceID = nil
end
 
local function FilterAura(element, _, data)
    if data.name and data.spellId == BOLSTER_ID then
        if not element.bolsterInstanceID then
            element.bolsterInstanceID = data.auraInstanceID
        end
 
        element.bolsterStacks = element.bolsterStacks + 1
        return element.bolsterStacks == 1
    end
end
 
local function PostUpdateButton(element, button)
    if not element.bolsterInstanceID then return end
    if element.bolsterInstanceID ~= button.auraInstanceID then return end
 
    button.Count:SetText(element.bolsterStacks)
end