It must be programmed like this since the addition of Singed getting a root as a reward for flinging his target into his adhesive is relatively new.
- Master Yi actives HIghlander, granting him slow immunity
- Singed Flings Master Yi towards Super Mega Adhesive
- Master Yi is now in Super Mega Adhesives area of effect
- Super Mega Adhesive will always try to apply slow while it persists in the field
- Super Mega Adhesive will only root if the game can confirm that Master Yi is within Super Mega Adhesive's AoE by checking if Master Yi is under it's debuff
- Since the debuff is not applied the game will not apply the additional rooting effect
I believe it would be programmed this way since confirming Singed's Fling target for a root would also require knowing 3 locations: target current location, target's new fling location, and Super Mega Adhesive's location and radius. This would have been the easiest solution to come up with to allow Singed's adhesive to gain the additional effect.