Highlander doesn't allow Skarner's E to mark Master Yi with crystal venom

Hello, and thank You for reading this. It happened in a game once, when i hit master yi with fracture, while his highlander was active. it didn't slow him and that was ok. however, there was no crystal venom on him, so i can't stun him. i don't know if this may be because the mark appears following the slow, or if it may be a mistake, however it seems to be not a little issue. Sorry for my bad english, i hope i could be of any help. thank You again for reading this