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

Lux E is taking 2x the mana it should
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I have to make this thread for bug visibility but I had seen it from this thread 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.
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