Skyrift BM mystic badge not working as described

[RagingPhenix]

https://ibb.co/i0fwaF

as you can see the effect clearly says "extends the duration on fulmination effect by 5 sec" but in actuality the effect produced is "lightning draw generates additional 5 sec fulmination".

 

if it worked as described then if i used blade storm (6 sec fulmination) then lightning draw i should have the existing 6 sec fulmination buff from blade storm extended by 5 sec for a total of 11 seconds. However the effect produced is that the 6 sec buff gets replaced by the 3+5 sec (8 sec) buff from lightning draw.