[SDPhantom]

If you hook the right functions, you can get the screenshot to activate perfectly each time.

local Hooked={};
local function OnFinished() RunBinding("SCREENSHOT"); end
 
hooksecurefunc("AchievementAlertFrame_SetUp",function(frame)
    if not Hooked[frame] then
        if frame.glow then frame.glow.supressGlow=true; end
        frame.animIn:HookScript("OnFinished",OnFinished);
        Hooked[frame]=true;
    end
end);

This catches when a dynamic achievement alert frame is shown and hooks the animIn animation to take a screenshot when it finishes fading in. This also tells the frames to suppress the glow animation so it doesn't interfere.