Very fun. Few things.
Entering a level you've already gotten a key for would be a soft lock if not for the return to hub button. Would be nice to show an outline of where the key was.
Choosing the wrong room from the hub consumes the magnet, so you have to go into the room and then port back out.
4th Room / Pipe: A large + beam attached to the ceiling by a tether. Not sure I solved this one correctly. I ended up throwing the magnet from off to the side and getting it to bounce off the switch. On a 2nd playthrough, I attached it to the ceiling, turned the switch off manually, then let the magnet fall down onto the switch. This feels like the intended solution.
Last Pipe: This took me far too long to complete. For the first 12 minutes or Trial and Error I did not realize that the rotating magnet "L" door would prevent the block from falling back to the bottom. My solution for this one also didn't feel right. I got crushed 3 times before finally squeaking through. Very little room for error, which makes me feel like I used an unintended solution. After a 2nd look, not sure how else to solve it.
There's some weird visual screen scrolling bug on the "Thanks for Playing" screen. A tile or two very close to the right side of the room causes the pixels to creep up and down a bit as you run back and forth across it.
The game desperately needs some sound. I think some audio cues would help massively.
All of that said, fantastic base. Can't wait to see it progress.