When leblanc use Sigil of malice and then rapidly use Distortion the mark of Sigil of malice doesn't trigger.
I believe that is because the missile speed is slightly not enough fast (by 0.1 sec) and that you can hit distortion before Sigil of malice reach the target, or the mark appears after the damage of Sigil of malice with a 0.1 sec delay which creates a window where you can hit Distortion without triggering the mark.
1. Use Sigil of malice on a target
2. Use Distortion on a target quickly after step 1.
3(? not confirmed). Use Auto attack command right after Sigil of malice to cancel animation in order to use Distortion faster.
4. Conditions : use it at a specific range between you and your target like in the replay.
Replay .gg link to find the game in which the bug happened : http://www.replay.gg/search/euw/somnusanima
The match in which the bug happened is this one : http://matchhistory.euw.leagueoflegends.com/en/#match-details/EUW1/2656127451/1?tab=overview
Screenshot on where to find the right game in replay.gg : https://gyazo.com/837a5e93aa552c72c3fa315fc51a5103
Time in-game when the bug happened : 2min 29sec to 2min 32sec
Proof that Distortion hit : Thunderlord proc after => Sigil of malice + Distortion + Auto attack (3Hits)
Proof that this isn't a visual bug : Thunderlod should have proc after => Sigil of malice + Distortion + Sigil of malice mark (Sigil of malice and distortion are enough to count as 3 hits, you can check it yourself in a custom game)
Rarety of the bug : Only happens to people that play leblanc a lot and use spells very quickly, so the better you are at using it fast the more chance you can trigger the bug.
Another bug (not in this game) which happens VERY RARELY, distortion can be recast mid-air before reaching the landing point (damage), probably because there is a small window (of 0.05sec or so) where you can cast distortion again to get back in the initiale position before dealing damage.
Note : This can't really be call a bug if my hypothesis are correct, but I don't think riot intended to let this happen.
Tell me if I can help, i'll do anything, the two bugs existed for a long time and has never been fixed.
VIT Nukeduck reproduced the bug aswell (2:37 - 2:42) : https://www.youtube.com/watch?v=X1HLZNppVRc
Edit 11st May : The bug seems to occur when Sigil of malice is cast at max range and the target move outside the range of Sigil after it has already been casted and then get hit by distortion, it looks like distortion is faster than Sigil of malice in this case.