Siphon Bullets doesn't give you a permanent stack if someone else finishes the enemy off.

[snipufin]

The way Siphon Bullets is worded right now ("if a siphoned target dies") makes it sound very much that it is intended to give you stacks for assists as well, but that is not the case.

 

[quiescentrabbitt]

I can confirm this bug is still in the game.