[Irongunner]

For now my workaround looks like this. Strangely the problem does not occur inside OnUpdate(..):

-- PostCastStart / PostChannelStart:
-- replace: castbar.Dummy.Icon:SetTexture(castbar.Icon:GetTexture())
-- by: castbar.Dummy.IconTexture = nil
     
-- PostCastSucceeded / PostCastStop / PostChannelStop:
-- add castbar.Dummy.Icon:SetTexture(castbar.Dummy.IconTexture)
       
local coreCastbarUpdate = function(castbar, elapsed)
    -- insert code from oUF core castbar.lua onUpdate(..) here
end
     
oUF_Iron.OnUpdate = function(castbar, elapsed)
    if castbar.Dummy.IconTexture == nil then
        local texture = castbar.Icon:GetTexture()
        if texture ~= nil then
            castbar.Dummy.IconTexture = texture
        end
    end
    coreCastbarUpdate(castbar, elapsed)
end
     
-- Register cb.OnUpdate = oUF_Iron.OnUpdate at castbar creation