Finally got around to trying it, and it works great! I'm not having the problem anymore except in a very weird specific case. It's not a big deal; I just thought you might want to know about it.
In the screenshot above, 9=impassable, 7=hole, 1=passable. The platform is the teal square with the white outline. For some reason, when the square above the platform is passable, the square on the left of the platform also becomes passable.
Thankfully, this fixes the problem for me: for whatever reason, if I make two squares to the left of the platform impassable, the problem goes away. The platform above is still touching a passable square, but when I test the game, I can't pass through the square to the left anymore.
Anyway, thanks so much for your work on this! I just figured you might want to know about the weird quirk in case it comes up again.