Hello! Thanks for playing and for your review, very much appreciated!
Sinister is, despite maybe appearing so, not at all random, and for those who speak Latin (I certainly do not) the solution is actually in the name: "sinister" means "left" in Latin, and that's the direction the monsters that are present in this maze always turn. Thus they rotate around the maze in a clockwise fashion. That knowledge is what Sinister tries to test, but I don't want people to bounce off a level because they feel they are not in control, so I will certainly consider whether the level is doing that job well. I might consider making Sinister a smaller level, both to give the player a better overview of the monster movement, and to enable quicker iteration, thus making failures less frustrating.
All the monsters currently present in Abysm are "wall huggers", exploring the space by constantly turning either left or right, depending on the type of monster. This is exactly for the reason you highlighted: puzzle games need to be deterministic. No monster will ever "chase" the player like they would in a typical action game. On a similar note, boulders always fall to the left if neither side is blocked.
Another thing to note maybe is that if you get stuck on a level you can choose "Skip level" and move on to the next for the time being, this way one can work on a few levels in parallel, something that I know I need to communicate better.