Lux's E bug when immediate detonation costs 140 mana instead of 70 !!

I have to make this thread for bug visibility but I had seen it from this thread

https://boards.na.leagueoflegends.com/en/c/gameplay-balance/rpQEMOhR-lux-mid-almost-unplayablebug-jinxylord-repertoir-please

Video included by Antenora from EUW in that thread that I linked

Lux´s E triggers double the mana cost when you detonate it immediately

Help!

Edit - for what I have experienced, the bug is also consistent, it´s not a matter of chance, it's 100% of the times.