Lux Ult+Flash trick

Demonstration: https://www.youtube.com/watch?v=Rfg1XgHyx_k

"Tutorial": https://www.youtube.com/watch?v=ZDW0NRle3no

I've tried to recreate this in a custom game for over 20 times since yesterday and to no success

I've tried different timings of flashes but my character always finishes the ult before flashing

I can ult-flash on lee sin pretty much 99% of the time

This has led to me believe, is it still possible to perform this trick or did riot patch it? if its still possible, what timing am i supposed to press flash?