Syndra got a mid-scope update with League of Legends Patch 12.19, and the community received it with open arms since it brought the champion back to relevance again after she started to fall behind the rest of the pack. But with new changes, there are, of course, new bugs.
Vandril, the diligent bug hunter, recently discovered a new Syndra interaction with Sion. The bug was, expectedly, discovered after her mid-scope update launched with Patch 12.19. Sion is unstoppable once he uses his ultimate, Unstoppable Onslaught. In this new interaction between Syndra and Sion, Syndra cant stop Sion from ulting, but she can redirect him and send him bouncing around Summoners Rift like a pinball.
Once Sion approaches Syndra with his Unstoppable Onslaught, all Syndra needs to do is use Scatter the Weak, and shell send him in a whole different direction.
This is presumably an unintended interaction between Syndra and Sion since Syndras Scatter the Weak would normally only do damage to Sion when using Unstoppable Onslaught and nothing more. On top of that, Sions ultimate is designed so the player navigating him is the only player who can change the direction of his ultimate.
Riot Games will hopefully fix this game-changing bug as soon as its brought to attention.