Working on a running theory to avoid the 15 sec bug every time as of the March 19th patch.
The idea is to get in melee range of the champs after they spawn, even let them hit you (so they have an agro target) then you can start attacking. Don't hit them with skyscale fireball before the timer starts. It might be coincidence but so far every time I've tried this method the event starts with the normal 10 minute timer.
Maybe I've been really lucky but I will keep testing it for more results.