I played through this game on hard and somehow never fought the maze monster (until the end obviously). Was I really lucky to never encounter him on my first visit?

Maze monster movement is based on character movement. So a little bit yeah.

Yeah your just lucky. Maze Monster actually just runs in one giant cycle around the maze independent of the player.

I think it takes roughly 1-2min for it to complete its cycle? Never actually timed it. But I have accidentally missed the cycle once and it was painful waiting for the spider to get back.

Thankfully for the genocide speedrun, the best spot to get him, allows you enough time to kill Noseferatu.