Ok, I have a nice self conversation
tl.add() is the better option, the only thing I don't understand is why it jumps from the first tween in the timeline back to opacity 0.5.... when the flip animation is completed...
https://codepen.io/destroy90210/pen/poBjery?editors=1011
Update: ok I think i got it, i need to add opacity: 1 also to the final state, because this would be used at the end as inlinestyles...
gsap.set(el_scalingTile, {
width: "100%",
height: "100vh",
top: 0,
left: 0,
position: "absolute",
opacity: 1 // <----
});