When the opponent has a Dryad of the Ilysian Grove in play, and then you play a Magus of the Moon, their lands keep their basic types.
Timestamps say that since Magus came into play 2nd, it takes priority and the non-basics should lose all basic land types.
Further info:
Modern League, Match ID: 255814466
In-game sequence that lead to events:
Game 2 of the match, I played Blood Moon. Then opponent plays Dryad of the Ilysian Grove. Their lands correctly get all basic types. They then play Leyline Binding to remove Blood Moon.
Next turn, I play Magus of the Moon. But their non-basic lands still had all types. Due to timestamps, and Magus entered the battlefield after the Dryad, their non-basic lands should be Mountains.
Timestamps say that since Magus came into play 2nd, it takes priority and the non-basics should lose all basic land types.
Further info:
Modern League, Match ID: 255814466
In-game sequence that lead to events:
Game 2 of the match, I played Blood Moon. Then opponent plays Dryad of the Ilysian Grove. Their lands correctly get all basic types. They then play Leyline Binding to remove Blood Moon.
Next turn, I play Magus of the Moon. But their non-basic lands still had all types. Due to timestamps, and Magus entered the battlefield after the Dryad, their non-basic lands should be Mountains.