I've made an instructional video a few weeks ago that shows how to prevent certain types of markings on the back of sleeves:

https://blogs.magicjudges.org/playerexperience/2019/04/30/how-not-to-sleeve-your-cards/

It's kinda topical right now with the Mythic Championship DQ, those markings were different but it does highlight the importance of having unmarked sleeves.

Please share this knowledge with the players if you come across such kind of markings in your tournaments. In my experience most players have no clue what causes them so just telling them to resleeve more often doesn't solve the problem. (I've had one player who I asked to resleeve between rounds buy new sleeves from the store, sleeve his deck, and before he could play the next round his new sleeves were already unacceptable.)